A post-processing apparatus connectable to an image forming apparatus which performs a post-processing on a sheet of a first type in a first post-processing mode, and performs the post-processing on a sheet of a second type having a light transmittance higher than the first type in a second post-processing mode, wherein, in a case where a job for performing the post-processing on a sheet of a third type is executed, the post-processing apparatus determines whether the sheet of the third type is detectable by the sheet detecting unit, operates in the first post-processing mode when the sheet of the third type is determined to be detectable by the sheet detecting unit, and operates in the second post-processing mode when the sheet of the third type is determined to be undetectable by the sheet detecting unit.