A sheet separating device is attached to a fixing device. The fixing device includes: a fixing member that is a rotary body having a heating unit; and a pressing member that is pressed against and is brought into contact with the fixing member. The fixing device conveys a sheet material that carries unfixed toner thereon through a nip section formed by the fixing member and the pressing member so as to fix the unfixed toner image to the sheet material. The sheet separating device includes: at least one nozzle member that discharges compressed gas from the direction of a nip outlet toward the nip section along the fixing member. The nozzle member is rotatably supported by a shaft member that is provided parallel to the fixing member and includes a gas flow passage through which compressed gas is supplied to the nozzle member.