To provide a data processing device which enables a client device to grasp a power mode of the data processing device without increasing the number of times of transmission and reception of packets between the data processing device and the client device.

An MFP 1 includes: a printer 11 which performs print processing according to a print request transmitted from a client device 3; a power control part 23 which supplies power to the printer 11 in a normal mode and stops the supply of power to the printer 11 in a power-saving mode; a network I/F 16 which returns a response packet including timing information indicating timing for returning to the normal mode when receiving an inquiry packet inquiring the timing for returning to the normal mode from the client device 3 during the power-saving mode.