PURPOSE: To provide a sheet detecting device which can detect a sheet made of a certain material and which can detect a sheet without making solid-contact with the sheet.

CONSTITUTION: There is provided a fluid discharge means 3 facing a conveyer passage 8 for conveying a sheet 7, for discharging air 19 having a predetermined pressure onto the outer surface of the sheet, and an arm 11 opposed to the fluid discharge means 3 and adapted to be operated by the pressure of air fed from the fluid discharge means 3, for controlling the turn-on and -off of a detecting circuit 30.