PURPOSE: To provide a chair which allow easy accommodation of sitting persons increased unexpectedly.

CONSTITUTION: A sitting part 1 of a chair is made up of a fixed sitting party formed integral with a backrest part 2 and a mobile sitting part 4 which is so arranged to cover at least a sitting surface of the fixed sitting part 3 while being separable from the fixed sitting part 3. Normally. the chair is used with the mobile sitting part 4 fitted into the fixed sitting part 3. When sitting persons are increased unexpectedly, the mobile sitting part 4 is pulled out of the fixed sitting part 3 to use thereby allowing the accommodation of sitting parsons increased quickly and easily.