Disclosed is a speed controller for a motor, which gives a normal rotation and constant speed output to an output cover when a driving shaft of the motor rotates in a normal direction and a normal rotation and speed-changed output to the output cover when the driving shaft of the motor rotates in a reverse direction, wherein when the restriction means are restricted to the driving shaft at constant speed and changed speed, at least one of the two restriction means is not restricted at any position, so that interference caused by a reverse input can be prevented although rotation is reversely input from the output cover. In the speed controller for motor preventing interference from reverse input according to the present invention, a key is formed to protrude on an outer circumference of the driving shaft; a pawl-mounting ring having a key groove with a greater width than the key is coupled to the key; a constant speed pawl and a speed changing pawl having opposite directions are mounted to the pawl-mounting ring so that lower portions of them extend to the key groove; and the pawl-mounting ring is frictionally fixed by an additionally fixed friction pin, whereby at least one of the constant speed pawl and the speed changing pawl is made lie down by the key according to a rotation direction of the driving shaft. Also, according to the present invention, the driving shaft has a polygonal outer circumference; a ball-mounting ring is coupled to the outer circumference, the ball-mounting ring including therein a constant speed ball and a speed changing ball spaced apart from each other by a predetermined distance so that upper and lower sides of them protrude; and the ball-mounting ring is frictionally fixed by an additionally fixed friction pin, whereby at least one of the constant speed ball and the speed changing ball escapes from the outer circumference according to a rotation direction of the driving shaft.