A post-processing apparatus according to an embodiment includes a processing tray, a post-processing section, a sensor, and a control section. A sheet supplied from an image forming apparatus is led into the processing tray through a lead-in path. The post-processing section applies post-processing to the sheet sent from the processing tray. The sensor detects an object passing a detection space in the lead-in path. The control section controls operation of the post-processing section based on a time in which the sensor detects the object.