To provide an automatic grinder which can accurately and quickly grind a shear blade, a cutting blade and a planer blade etc., is small-sized, lightweight and easily portable, and assures an automatic grinding.

A stopper 4f is screwed between mounting plates which are mounted at both ends of a standing-up plate 4n on a guide plate 4a, a grinding part 4 to which is reciprocatably mounted a slide plate 4h having a presser plate 4k which presses an object to be ground with which is connected a transversial shaft to which is mounted a switch frame body 4e which is provided, at the center, with a switch which changeovers a driving motor provided with a first, second and third belts which transmit the rotation of the motor to an upper, middle and lower pulleys secured to the rotary shaft of the driving motor, the first belt is stretched over the first pulley and a first adjuster pulley is provided, the second belt is stretched over the second pulley and a second adjuster pulley is provided and the third belt is stretcher over the fourth pulley and a third adjuster pulley is provided, a fourth belt is stretched over the fourth pulley and a third pulley, is mounted to a driving part 2 to which is mounted a rotary grindstone 7d adjustable by an adjuster member is mounted at the lower end of the rotary shaft 6 of the third pulley.