According to one embodiment of the present disclosure, there is provided a substrate processing apparatus comprising: a supply passage through which fluid supplied to a substrate flows; and a foreign substance detector including a channel forming part forming a portion of the supply passage, a light projector irradiating light to the channel forming part, and a light receiver receiving light emitted from the channel forming part as a result of irradiating light to the channel forming part by the light projector, the foreign substance detector being configured to detect a foreign substance in the fluid based on a signal obtained by the light that the light receiver receives, wherein the light projector and the light receiver in the foreign substance detector are disposed in areas that are not opposite to each other in areas in upper, lower, left, right, front and rear directions of the channel forming part.