A throttle opening detector for internal combustion engine is composed of a slide contact device which has first and second contacts with potential periodically varying in pulse form corresponding to throttle opening variation, phase difference of pulses beteeen said first and second contacts being a fourth of the period, and of a throttle opening reading device which reads out throttle opening from the potential variation in the first and second contacts and variations of exclusive-OR of potential signals in both contacts. The throttle opening detector enables to detect throttle opening precisely using a cheap micro-computer in 2 bits and to carry out the initial adjustment automatically.