In a substrate processing apparatus, an optical sensor is provided at a hand that transports a substrate to a processing unit, and an optical fiber is provided at a fixed member that has a certain positional relationship with a spin chuck in the processing unit. When the hand has a predetermined positional relationship with the spin chuck in the processing unit, the light emitted from a first light emitter of the optical sensor is received by a second light receiver of the optical fiber and guided to a second light emitter of the optical fiber, and the light emitted from the second light emitter is received by a first light receiver. A light receiving signal corresponding to an amount of light received by the first light receiver is output from the optical sensor.