To provide a portable terminal device charging system, portable terminal device charger, portable terminal device, portable terminal device charging method, capable of suppressing lowering of performance and a charging speed and a temperature rise of the portable terminal device even in charging the portable terminal device while using the portable terminal device.

The portable terminal device charging system includes: a portable terminal device 11 that has a terminal side processing section 13 for implementing any processing and a charger 12 that charges the portable terminal device 11. The charger 12 includes an alternative processing section 14 that implements at least a part of processing to be implemented during a charging period of the portable terminal device 11 by the terminal side processing unit 13 during a charging period as an alternative to the terminal side processing unit 13.