Provided is a DC/DC converter for a switching power supply device including a plurality of DC/DC converters which include a master DC/DC converter and a plurality of slave DC/DC converters and have output terminals connected in parallel to each other. The DC/DC converter includes: first and second terminals that are used to cascade the plurality of DC/DC converters in a line; a control circuit that determines that a host DC/DC converter is the master DC/DC converter when a predetermined voltage is detected at the second terminal and determines that the host DC/DC converter is the slave DC/DC converter when the predetermined voltage is not detected; and that controls a current output from the second terminal based on whether the host DC/DC converter is the master DC/DC converter or the slave DC/DC converter.