The brushless motor driving circuit includes a peak hold circuit that outputs a second voltage obtained by holding a peak of a first voltage that is based on a detected voltage responsive to a voltage drop in the detecting resistor. The brushless motor driving circuit includes a filter circuit that outputs a third voltage, which is a direct-current component of the second voltage. The brushless motor driving circuit includes a differential voltage detecting circuit that outputs a first differential voltage between the second voltage and the third voltage. The brushless motor driving circuit includes a computing circuit that calculates a second differential voltage between the analog voltage and a fourth voltage responsive to the first differential voltage, and outputs a speed control signal responsive to the second differential voltage.