Throttle valve opened amount calculator

A calculator for calculating an opened amount of a throttle valve with high accuracy regardless of voltage fluctuation and noise. The calculator includes a valve sensor (33) for detecting the opened amount of the throttle valve to generate a detection voltage. A memory (24) stores an initial zero point detection value. A calculation circuit (22) obtains the difference between the detection voltage and the initial zero point detection value and calculates the opened amount of the throttle valve with the difference. When a plurality of detection voltages that arc less than the initial zero point detection value are generated, the calculation circuit (22) calculates a new zero point detection value that is greater than a smallest one of the plurality of detection voltages by a predetermined value and updates the initial zero point detection value with the new zero point detection value.