A control device of an insulation type DC/DC converter includes a shunt regulator that detects an error in output voltage at the secondary side of a transformer, a photocoupler that transmits the detected error voltage to the primary side of the transformer, and a DPWM control unit on the primary side that generates a control pulse signal having a pulse width at a duty ratio based on the error voltage. The DPWM control unit includes an A/D conversion circuit, an A/D output stabilization circuit, a dither circuit, and a DPWM circuit. The A/D output stabilization circuit is provided after the A/D conversion circuit, and the output end of the A/D output stabilization circuit is connected to the input end of the dither circuit.