In a control device applied to an insulated DC-DC converter including a transformer and drive switches connected to a input winding of the transformer, the control device operates the drive switches to control an output voltage output via the output winding to a target voltage. Furthermore, the control device includes a voltage acquiring unit acquiring the output voltage and a frequency setting unit setting a switching frequency for the drive switches based on the output voltage acquired by the voltage acquiring unit. The frequency setting unit sets, for the switching frequency, a value increasing with a value of the acquired output voltage, and sets the switching frequency such that an increase in the value of the output voltage increases a frequency increase rate corresponding to an amount of increase in the switching frequency per unit amount of increase in the output voltage.