A data transfer control device includes an OUT-transfer transmitter circuit which transmits OUT data by driving a serial signal line, a clock-transfer transmitter circuit which transmits a clock signal CLK by driving a serial signal line, a PLL circuit which generates the clock signal CLK, and a power-down setting circuit which sets a power-down mode. In a first power-down mode, the OUT-transfer transmitter circuit is set to the power-down mode, and the clock-transfer transmitter circuit is set to the power-down mode to stop a system clock signal of a target-side data transfer control device. In a second power-down mode, the OUT-transfer transmitter circuit is set to the power-down mode without setting the clock-transfer transmitter circuit to the power-down mode.