To provide a motor-driven disk brake capable of preventing damage to boots to be caused by fluctuation in the inside air pressure.

The motor-driven disk brake comprises a caliper main body 100, a drive motor 19 installed in the caliper main body 100, a conversion mechanism 21 which converts the rotational motion of the drive motor 19 into the rectilinear motion, a piston 40 which is forced in the rectilinear motion by the conversion mechanism 21 housed freely movably in the caliper main body 100, and elastic boots 99 which block the opening part between the piston 40 and the caliper main body 100. The motor-driven disk brake further comprises an atmospheric communicating means 112 to make the inside space 114 which is surrounded by the caliper main body 100, piston 40 and boots 99 and houses the drive motor 19 and the conversion mechanism 21 therein, to communicate with the atmosphere.