A power supply regulation circuit is provided which can achieve a preferable constant current drooping characteristic without the need for a constant current control circuit on the secondary side. During a constant current operation, a switching element (1) is turned on so as to keep constant the on duty of secondary current passing through a secondary winding (110B) of a transformer (110), so that a constant current drooping characteristic is achieved. Further, at this moment, a current limit dictating the maximum value of drain current (ID) passing through the switching element (1) is changed between first and second levels during the on time of the switching element such that the drain current (ID) has a constant peak value.