A control circuit can be provided with a comparator detecting a turning-off of a semiconductor switch from a voltage generated in an auxiliary winding of a transformer and inputted to a zero current detecting terminal, a timer outputting a signal after a specified length of time from the time at which the turning-off of the semiconductor switch is detected, and a comparator detecting from voltages inputted to a current detecting terminal a second voltage signal different from an ordinary first voltage signal inputted when the semiconductor switch is made turned-on. Thus, the second voltage signal supplied from mode switching circuits as an externally inputted mode switching signal can be made also detected by the existing current detecting terminal. By detecting the second voltage signal after the specified length of time from the turning-off of the semiconductor switch, switching of an operation mode becomes possible without newly providing any specialized terminal.