A wet brake device comprising: a brake case in which a rotational shaft is rotatably fitted; a plurality of rotating side brake disks arranged outside in a radial direction from said rotational shaft in said brake case and rotating together with said rotational shaft; a plurality of non-rotating side brake disks arranged in a non-rotating state in said brake case while alternately overlapping with each of said rotating side brake disks; a brake piston which applies a braking force to said rotational shaft by pressing said rotating side brake disk and said non-rotating side brake disk and frictionally engaging them with each other; a lubricant oil inlet port (40, 40') provided in said brake case and into which lubricant oil flows; and a lubricant oil outlet port provided in said brake case and from which said lubricant oil inside flows out; characterized in that: said rotational shaft is configured to extend in the vertical direction in said brake case; said non-rotating side brake disk is arranged opposite to said rotating side brake disk in the vertical direction; a frictional material which generates a braking force between said rotating side brake disk and said non-rotating side brake disk is provided only on said non-rotating side brake disk; said lubricant oil inlet port (40, 40') is to have said lubricant oil flow from the outside of said brake case to outer diameter side portions of said rotating side brake disk and said non-rotating side brake disk; said lubricant oil outlet port is to have said lubricant oil discharged from inner diameter side portions of said rotating side brake disk and said non-rotating side brake disk flow out to the outside of said brake case; and an oil groove for having said lubricant oil flow from the outer diameter side to the inner diameter side of said rotating side brake disk and said non-rotating side brake disk is provided in said frictional material of said non-rotating side brake disk. a brake case in which a rotational shaft is rotatably fitted; a plurality of rotating side brake disks arranged outside in a radial direction from said rotational shaft in said brake case and rotating together with said rotational shaft; a plurality of non-rotating side brake disks arranged in a non-rotating state in said brake case while alternately overlapping with each of said rotating side brake disks; a brake piston which applies a braking force to said rotational shaft by pressing said rotating side brake disk and said non-rotating side brake disk and frictionally engaging them with each other; a lubricant oil inlet port (40, 40') provided in said brake case and into which lubricant oil flows; and a lubricant oil outlet port provided in said brake case and from which said lubricant oil inside flows out; characterized in that: said rotational shaft is configured to extend in the vertical direction in said brake case; said non-rotating side brake disk is arranged opposite to said rotating side brake disk in the vertical direction; a frictional material which generates a braking force between said rotating side brake disk and said non-rotating side brake disk is provided only on said non-rotating side brake disk; said lubricant oil inlet port (40, 40') is to have said lubricant oil flow from the outside of said brake case to outer diameter side portions of said rotating side brake disk and said non-rotating side brake disk; said lubricant oil outlet port is to have said lubricant oil discharged from inner diameter side portions of said rotating side brake disk and said non-rotating side brake disk flow out to the outside of said brake case; and an oil groove for having said lubricant oil flow from the outer diameter side to the inner diameter side of said rotating side brake disk and said non-rotating side brake disk is provided in said frictional material of said non-rotating side brake disk.