To further efficiently reduce power consumption by efficiently controlling a power save mode of a serial interface part.

This hard disk drive HDD selectively transits the serial interface part 236 to the power save mode through another circuit. The HDD receives a command from a host 51, and sets the serial interface part 236 to the power save mode at least for a partial period where data transmission and receipt by serial data communication with the host 51 are not performed in the command execution sequence. The HDD also transits the serial interface part 236 to the power save mode in response to the completion of command execution.