To achieve a structure reducing the size and the weight and preventing the reaction force accompanying breaking from acting on a back plate.

A first extension lever 33 and a second extension lever 34 are supported for rocking displacement around a pivot 31. The tip part of the first extension lever 33 is engaged with a web 7a of one brake shoe 4a in a depressible state of the web 7a outward in a radial direction. The tip part of the second extension lever 34 is engaged with a base end of a strut 32 in a depressible state of the strut 32 outward in the radial direction. With a transmission mechanism 56 forming a drive device 27a, the first extension lever 33 and the second extension lever 34 are displaced rockingly in opposite directions to each other.