An electronic flow meter includes a magnetic sensor, phase-A and phase-B drivers, a phase-A comparator, a phase-B comparator, a two-phase encoder, and a rotational speed timer. The two-phase encoder determines a normal/reverse direction where the impeller is rotated based on event signals sampled by the phase-A and the phase-B comparators, respectively, causing a counter to count up or to count down according to the normal/reverse direction as determined, thereby outputting an event count signal. The rotational speed timer calculates a rotational frequency of the impeller from the event count signal outputted by the two-phase encoder and sets frequencies of respective sampling signals of the phase-A and the phase-B comparators at a time when the rotational frequency of the impeller is measured next, and respective drive periods of the phase-A and the phase-B drivers in accordance with the rotational frequency of the impeller as calculated.