A speed controller for self-closing sliding doors for controlling the closing speed of an automatic self-closing sliding door, characterized by comprising a generator provided in conjunction with the sliding door, one-way motion transmitting means for converting a linear motion occurring when the sliding door is closed to a rotating motion to transmit it to the generator, an electric resistor connected to the output of the generator, and speed changing means for decreasing the resistance of the electric resistor and thereby changing the closing speed of the sliding door from a first predetermined speed to a second predetermined speed slower than the first one in a predetermined position before the closed position of the sliding door.