A constant dc current supplying circuit comprises a dc voltage supply (20), an inverter (30) providing an ac voltage from said dc voltage, and an ac-to-dc converter (50) providing a dc current from the inverter output and supplying the resulting dc current to a load (10). The inverter (30) includes a switching element (31) driven to turn on and off in order to repetetively interrupt the dc voltage for producing the dc current of which level is determined by on-off time ratio of the switching element (31). A comparator (100) is included to compare a level of the dc current supplied to the load with a predetermined reference level so as to provide a first output and a second output when the dc current level is below and above the reference level. Also included in the circuit is a feedback controller (110) which generates first and second feedback control signals, respectively in response to the first and second outputs from the comparator (100). The first and second control signals are defined as a train of pulses of varying duty ratio in order to increase and decrease on-off time ratio of the switching element (31) depending upon the sensed output dc current level, whereby controlling to gradually increase and decrease the output dc current towards the reference level for maintaining the dc current thereat.
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