To perform on-off control from outside under the condition of being insulated from a primary circuit, in a simple constitution which does not use an internal auxiliary power supply, an external power supply or the like.

This switching power unit is equipped with an on/off control circuit 12 comprising an auxiliary winding 2d which is wound on a transformer 2, being insulated from a primary winding 2a, and on/off control terminals 13 and 14 which open or short-circuit both ends of this auxiliary winding 2d. Moreover, this unit is equipped with a resistor 4, which stops the action of a MOSFET 3 by dropping an output voltage Vout at short circuiting of the auxiliary winding 2d. Hereby, the duty ratio of a pulse drive signal given to a MOSFET 3 drops to a very low level by short circuiting both ends of the auxiliary winding 2d, making use of on/off control terminals 13 and 14, whereby the output voltage Vout can be made to drop.