To provide a chair with a function to detect seating posture which can monitor seating posture when a child studies at a study desk or a worker works at an office desk, a work station, or a visual display terminal.

A contact sensor 5A, 5B for a seated person T is provided at a prescribed position either or a rear part of a seat part 3 or on a backrest part 4, or on the both of them. The contact sensor 5A, 5B monitors whether either the buttocks or the back of the seated person, or both of them are in contact with the contact sensor or not. When either the buttocks or the back of the seated person loses contact with the contact sensor for a certain period of time, alarm will set off.