WET BRAKE DEVICE

A plurality of rotating side brake disks (47) and a plurality of non-rotating side brake disks (48) are arranged outside in the radial direction from a motor shaft (25) in a brake case (32) while alternately overlapping with each other. A frictional material (48B) is provided on the non-rotating side brake disk (48), and an oil groove (48F) is provided in a frictional engaging surface (48E) of this frictional material (48B). It is configured such that, when a braking force is applied by a brake piston (49) to the motor shaft (25), lubricant oil (59) flowing into the brake case (32) from a lubricant oil inlet port (40) flows from the outer diameter side to the inner diameter side between the rotating side brake disk (47) and the non-rotating side brake disk (48) only through the oil groove (48F). As a result, the lubricant oil (59) having flowed into the brake case (32) efficiently cools the frictional engaging surface (48E).