The present invention provides a treadmill with an automatic speed control function, including: a body having a belt for supporting an exerciser; an exerciser detecting portion installed in a predetermined area of the body to detect movement of the exerciser; a driving motor coupled to the body to drive the belt; a control portion for generating a control signal for adjusting a rotation speed of the driving motor based on a signal received from the exerciser detecting portion; and a motor driving portion for adjusting the rotation speed of the driving motor according to the control signal received from the control portion, wherein a provision braking torque provided corresponding to the control signal is varied as the rotation speed of the driving motor is reduced. The treadmill quickly follows acceleration or deceleration of an exerciser, realizes a feeling like what an exerciser has while exercising on the ground to thereby improve an exerciser's exercising feeling, accepts various exercising patterns of an exerciser, resolves a problem in that a motor driving portion is tripped due to a load caused by quick deceleration, and pre-processes measured values of an exerciser's position to resolve a problem in that a speed of a belt can not be controlled due to measurement errors contained in measured values.