To prevent damages on a brake frictional plate by forming a groove in a contact part of a casing to be in contact with an outer edge of the brake frictional plate in a hydraulic motor, not coming into contact with an outer edge of the brake frictional plate but for achieving surface contact.

A hydraulic motor parking brake includes a casing 11, a rotation shaft 12, a cylinder block 14, a movable brake disk 22 supported by an outer circumference 14a of the cylinder block 14, and a brake piston 25. An annular groove 30 is formed on a contact face 29 of the casing 11 where the movable brake disk 22 is brought into contact with.