A capacitance detecting circuit includes a constant current source supplying a constant current to a voltage detection point, a time measuring unit measuring time of charge until the voltage of the voltage detection point reaches a prescribed voltage, a storage unit and an operation unit. The storage unit stores first data obtained in advance concerning the time of charge of a capacitance element having a known capacitance value, and second data obtained in advance concerning the time of charge of a capacitance component. The operation unit generates third data representing the time of charge of an object of detection from a difference between the first measurement value of the time measuring unit when the capacitance component is connected to the voltage detection point and the second data, and based on the ratio between the third data and the first data and the known capacitance value, calculates the capacitance value of the object of detection.