Disclosed herein is an electronic disc brake in which driving force of a motor may be transmitted to a gear unit in a serial manner. The electronic disc brake which has a carrier, a caliper housing, a pair of pad plates disposed at both sides of a disc, and a piston installed within a cylinder part and pressing one of the pair of pad plates toward the disc, includes a spindle member passing through the rear wall of the caliper housing and rotated within the cylinder part, a nut member screw-connected to the spindle member and pressing the piston or releasing pressing of the piston, a motor generating driving force, and a cycloid reducer amplifying the driving force generated from the motor and transmitting the amplified driving force to the spindle member, and the motor, the cycloid reducer and the spindle member are coaxially connected in series.