A control device of a multi-phase converter having N converter circuits connected in parallel includes: a determination unit configured to determine each share ratio of the N converter circuits to unevenly share input current to the multi-phase converter among the N converter circuits such that a conversion efficiency indicating a ratio of output power from the multi-phase converter with respect to input power to the multi-phase converter is higher in the case where the input current is unevenly shared by the N converter circuits compared to the case where the input current is evenly shared by the N converter circuits when the number of driven converter circuits is one or more and N−1 or less and the start condition is satisfied; and a diagnosis unit configured to diagnose an abnormality of the N converter circuits when the N converter circuits are driven in accordance with the determined share ratios.