Kagami no Kuni no Harisugawa

Kagami no Kuni no Harisugawa (鏡の国の針栖川) is a manga written and illustrated by Kano Yasuhiro and serialized in Shōnen Jump.

Plot
Harisugawa is your everyday average high school boy. He has a secret crush on Satomi Mao whom he saved her from a car accident when they were in elementary school, thus becoming her Life Saver. Time passed and they are now in high school while still maintaining their just friend status quo. One day when Mao buys a dusty old mirror from a thrift store, she tries to show it to Harisugawa when she is almost run over by a car again. Trying to save her, Harisugawa jumps in, but when he regains his consciousness, he realize that he is inside of the mirror Mao had bought. How will Harisugawa leave from this strange world?