A DC/DC converter (1, 2, 3) for a multi-phase switching power supply device which includes a plurality of DC/DC converters (1, 2, 3) including a master DC/DC converter and a plurality of slave DC/DC converters (2, 3) that are connected in parallel to each other and operate in different phases, each DC/DC converter comprising: a DC/DC converter unit; first and second terminals for connecting the plurality of DC/DC converters (1, 2, 3) in a current connection line; a phase detector configured to detect the phase of the DC/DC converter, and comprising a control circuit configured to determine whether the DC/DC converter is the master DC/DC converter or one of the slave DC/DC converters (2, 3) based on a voltage of at least one of the first and second terminals, and wherein, when it is determined that the DC/DC converter is the master DC/DC converter, calculate a number of operating converters among the plurality of DC/DC converters connected to each other on the basis of a current which flows to the first terminal and a current which flows from the second terminal; and wherein, when it is determined that the DC/DC converter s one of the slave DC/DC converters (2, 3), detect the number of converters which are operating in the switching power supply device of the DC/DC converter on the basis of a PWM modulated clock signal from the master DC/ DC converter; a phase holder configured to hold the detected phase and output a phase holding signal indicating the detected phase; a clock generator configured to select on the basis of the phase holding signal, a clock corresponding to the phase of the DC/DC converter from multi-phase clocks which are generated from a reference clock, and outputs the selected clock to the DC/DC converter unit; wherein the DC/DC converter unit performs a switching operation on the basis of the selected clock output from the clock generator. a DC/DC converter unit; first and second terminals for connecting the plurality of DC/DC converters (1, 2, 3) in a current connection line; a phase detector configured to detect the phase of the DC/DC converter, and comprising a control circuit configured to determine whether the DC/DC converter is the master DC/DC converter or one of the slave DC/DC converters (2, 3) based on a voltage of at least one of the first and second terminals, and wherein, when it is determined that the DC/DC converter is the master DC/DC converter, calculate a number of operating converters among the plurality of DC/DC converters connected to each other on the basis of a current which flows to the first terminal and a current which flows from the second terminal; and wherein, when it is determined that the DC/DC converter s one of the slave DC/DC converters (2, 3), detect the number of converters which are operating in the switching power supply device of the DC/DC converter on the basis of a PWM modulated clock signal from the master DC/ DC converter; a phase holder configured to hold the detected phase and output a phase holding signal indicating the detected phase; a clock generator configured to select on the basis of the phase holding signal, a clock corresponding to the phase of the DC/DC converter from multi-phase clocks which are generated from a reference clock, and outputs the selected clock to the DC/DC converter unit; wherein the DC/DC converter unit performs a switching operation on the basis of the selected clock output from the clock generator.