A current source is provided between a first p channel MOS transistor and a ground node, and a current/voltage converting element is provided isolatedly from the current source between the ground node and a second p channel MOS transistor having a conductance coefficient sufficiently larger than that of the first MOS transistor. The second MOS transistor is connected through a resistive element to an external power supply node. A voltage produced by the current/voltage converting element is converted into current by a voltage/current converting portion. Thus, constant current free from both vibration and a deadlock phenomenon and with small external power supply voltage dependency is supplied.