CONSTANT CURRENT CIRCUIT

PROBLEM TO BE SOLVED: To provide a constant current circuit that can be miniaturized and which has less heat generation. SOLUTION: This constant current circuit 10 is provided with a switching transistor Tr (FET) that, using as an input a DC voltage VIN obtained from a commercial power 'e' through an AC adapter 1, performs the switching of the DC voltage VIN; a filter circuit 3 that smoothes the output of the switching transistor Tr; a low-resistance element R1 for detecting a current inserted in series to a current path that supplies the output current 'i' coming out of the filter circuit 3 to a battery Bat; an amplifying circuit 5 that amplifies the voltage drop Vd of the low-resistance element R1; and a voltage-control circuit 7 that compares an output voltage Vo of the amplifying circuit 5 with a reference voltage Vref and controls the switching duty of a switching pulse wave P that is inputted to the gate G of the switching transistor Tr by the difference. The current control of the switching system enables the constant current circuit 10 to be miniaturized with proper efficiency and less heat generation, and makes it particularly well fit to the charging of a battery of a portable electronic device.