When a replaceable unit is mounted to a main body, if a main body identifier reference value stored in a local memory of the replaceable unit is different from the main body identifier, the main body rewrites the main body identifier reference value, writes a life counter value stored in the main body memory into the local memory as a master counter reference value, and writes a life counter value stored in the local memory separately into the local memory as a life counter reference value. The main body increments the life counter values due to an action, and determines a life count value of the replaceable unit on the basis of a difference between the life counter value in the main body memory and the master counter reference value and a difference between the life counter value in the local memory and the life counter reference value.