In an insulated double gate FET, the threshold voltage during the operation of a transient response thereof is enabled to be arbitrarily and accurately controlled by a method that includes applying a first input signal intended to perform an ordinary logic operation to one of the gate electrodes thereof and applying, in response to this signal, a second signal that has a signal-level temporal-change direction as the first input signal and has at least one of the low level and the high level thereof shifted by a predetermined magnitude or endowed with a predetermined time difference or has the time slower or faster signal level change of the signal to the other gate electrode.