A method for reducing power consumption in an electronic control unit (ECU) equipped with an Ethernet communication function and mounted in a vehicle include initializing a physical layer upon restarting of the physical layer and setting a transmission mode to a data mode. The method includes generating a clock signal having a first frequency for Ethernet communication, checking presence or absence of a normal frame to be transmitted, checking presence or absence of an idle frame based on a reception signal symbol, and determining whether to change the frequency of the clock signal based on checking results in the checking of presence or absence of a normal frame and the checking of presence or absence of an idle frame.