Whether a human body is touching or not is detected highly accurately at high speed. Switch SW1 is turned on to charge condenser Ca by power supply Vcc. The switch SW1 is turned off to transfer the electric charge charged in the condenser Ca to condensers Cs and Cx. The switches SW2 and SW3 are repeatedly turned on and off for a predetermined short time, and thus the electric charge charged in the condenser Ca is slowly discharged through resistor R. The unknown charging voltage Vx of the condenser Cx is compared with the reference voltage Vref by comparator Comp, and the unknown capacitance of the condenser Cx is computed based on the number of times to count turning on and off until it is smaller than the reference voltage Vref. When the computed unknown capacitance of the condenser Cx is the value near the capacitance of a human body, it is determined that a human body is touching. The invention can be applied to a touch sensor.