To provide a motor driving circuit small-sized, lightweight, and capable of keeping a source voltage at a specified value by the use of a few components.

If a DC motor M rotates quickly when the DC motor M rotates in the reverse direction, and the voltage of a junction P tries to be a voltage value not lower than (E+α) V which was set beforehand by generation action, the voltage increases the terminal-to-terminal voltage of a capacitor C2 via a source voltage detecting circuit 20 composed of Zener diodes D5-D7 and a resistor R5, and a PWM(pulse width modulation) circuit 24 increases a duty factor for a forward direction of rotation. Since the reverse rotating speed of the DC motor M decreases, and the voltage of the junction P is kept equal to or lower than (E+α) V, no stress is applied to the components of a power circuit 12 and an H bridge circuit 14, and use of radiating fins, etc., also becomes unnecessary.