A reflective sheet detector includes first and second guide plates constituting a sheet travelling path, an emitter, a receiver and a reflecting portion. The emitter is disposed on a first side of the sheet travelling path and close to a first input opening of the first guide plate. The receiver is disposed on the first side and close to a first output opening of the first guide plate. The reflecting portion is disposed on a second side of the sheet travelling path opposite the first side and is close to the second guide plate. The emitter outputs a signal to the reflecting portion through the first input opening and a second input opening of the second guide plate, and the reflecting portion reflects the signal to the receiver through a second output opening of the second guide plate and the first output opening.