Read manga: Shishunki No Iron Maiden

manga
Summary:

Mayama Haruki and Himemiya Tsukiko have been childhood friends and neighbors for as long as they can remember, but after a girls-only meeting in fourth grade, Himemiya distanced herself from Mayama. Now in their 2nd year of middle school, Mayama has become an ero-freak who specializes in harassing Himemiya.Determined to unravel the greatest secret of girls, what happens on "that day", Mayama follows Himemiya and uncovers...!("that day" for girls may not be what you think it is)-from mangaupdates-

Chapters (41)
Aug 25, 2019
Chapter 41
Aug 25, 2019
Chapter 40 : Secret Base
Aug 25, 2019
Chapter 39 : The Great Escape
Aug 25, 2019
Chapter 38 : Remember!
Aug 25, 2019
Chapter 37 : The Truth Behind The Metamorphosis
Aug 25, 2019
Chapter 36 : Observing The Observation Diary
Aug 25, 2019
Chapter 35 : Read Online
Aug 25, 2019
Chapter 34 : The Princess, Captured...!?
Aug 25, 2019
Chapter 33 : Uneasy Days
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Chapter 32 : Read Online
Aug 25, 2019
Chapter 31 : Fuck You!!
Aug 25, 2019
Chapter 30 : Bye Bye Xxx
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Chapter 29 : First Confession
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Chapter 28 : From Pure White To Purity!
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Chapter 27 : The Second Album
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Chapter 26 : Charm Maiden
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Chapter 25 : If The Tears Poured
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Chapter 24 : Run Young Man!!
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Chapter 23 : The Confession
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Chapter 22 : Coward
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Chapter 21 : The Passing Days
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Chapter 20 : Nonoa's Gloomy Day
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Chapter 19 : History's Biggest Battle Plan
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Chapter 18 : It's Show Time!!
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Chapter 17 : Ptm: Peeping Teacher Minowa
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Chapter 16 : Another Observation...
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Chapter 15 : Thin Ice
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Chapter 14 : Coming Out!
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Chapter 13 : Howling Boy!
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Chapter 12 : Looking For The Culprit
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Chapter 11 : A Play Back Of That Day
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Chapter 10 : Secret X Secret
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Chapter 9 : Iron Protrusions!
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Chapter 8 : That Girl's Observation
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Chapter 7 : I'm Nonoa
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Chapter 6 : A Close Call
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Chapter 5 : That... Has Come Out
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Chapter 4 : Iron Will
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Chapter 3 : Not Eating What Is Set Before You
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Chapter 2 : Tsukiko's Secret
Aug 25, 2019
Chapter 1 : 'that Day' Of Hers