noun
exchange diary; shared diary between friends
A notebook passed back and forth between friends (often schoolgirls), where each person writes entries in turn. The term is culturally specific and widely understood in Japan.
中学生のとき、友達と交換日記をしていた。
When I was in junior high, I kept an exchange diary with a friend.
交換日記
An exchange diary is a good way to understand each other's feelings.