A processing device capable of performing appropriate processing on a set platen, and a platen are provided. A platen that is used in a printer and at least one of a pretreatment device and a post-treatment device is provided with a platen identification information portion that identifies the platen. A code reader receives platen identification information that identifies the platen, from the platen identification information portion. The printer and at least one of the pretreatment device and the post-treatment device perform processing on the basis of the platen identification information received by the code reader.