Post-processing apparatus and control method

A post-processing apparatus is connected to an image forming apparatus that forms an image on a sheet. The post-processing apparatus includes a post-processor, a sensor, and a controller. The post-processor performs post-processing on the sheet. The sensor is provided in a space where the post-processor operates and provides an output based on a presence of a foreign substance in the space. The controller acquires a first output value from the sensor prior to the image forming apparatus forming the image on the sheet, acquires a second output value from the sensor after the image forming apparatus forms the image on the sheet and before the post-processor performs the post-processing on the sheet, and determines whether or not the foreign substance is present in the space based on the first output value and the second output value.