A trouble determining apparatus for a chopper type DC boosting circuit capable of determining whether or not a switching device is in trouble securely and quickly and when the switching circuit is in trouble, preventing other circuit from being in a trouble. Upon determining a short-circuit trouble of a transistor Q, a boosting circuit control unit 22 is turned OFF a power relay RL and then determines whether or not a voltage VP1 of a connecting point P1 rises over a first set voltage VS1 by the time when a first predetermined time t1 elapses since an ignition switch IG is turned ON with the transistor Q set to OFF. Upon determining an open trouble of the transistor Q, the boosting circuit control unit 22 determines whether or not the voltage VP1 of the connecting point P1 falls below a second set voltage by the time when a second predetermined time t2 elapses since the transistor Q is turned ON with the power relay RL set to OFF and whether or not the voltage VP1 falls from a state before the transistor Q is turned ON by more than a third set voltage.