PURPOSE:To improve measuring accuracy, by performing conversion of a physical quantity to a frequency signal corresponding to a voltage value, thereafter performing conversion to a specified digital signal, performing a specified computation, and measuring the physical quantity. CONSTITUTION:In the case of a zero measuring mode, since time period t1 until the charging voltage of a capacitor Cf reaches an input measuring voltage is proportional to the input measuring voltage, a pulse signal whose frequency is proportional to the measuring voltage is obtained from the output of an FF Q1. Meanwhile, by counting the clock pulses from a reference clock generating circuit 6, a charging and discharging time T1 of a capacitor C can be measured. After the charging and discharging time T1 has been obtained, an E1 measuring mode is set. Then a charging and discharging time T2 of the capacitor C is measured. The input measuring voltage is obtained from the difference T2-T1, i.e. the difference between the charging and discharging times of the capacitor C in the E1 and zero measuring modes. In this method, the measuring accuracy can be improved.
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