To provide a wet brake device capable of striking a balance between the securement of durability of a brake plate and the securement of braking performance.

Between a brake case 32 and a brake piston 49, a brake pressure adjusting oil chamber 58 is provided into which lubricating oil 59 in a pressurized state flows. Further, by the lubricating oil 59 flowing into the brake pressure adjusting oil chamber 58, force opposed to urging force of a spring member 53 is applied to the brake piston 49. Thereby, without reducing the urging force of the spring member 53, it is possible to prevent application of sudden excessive force to a rotation side brake plate 47 and a non-rotation side brake plate 48 from the brake piston 49 during braking, and both of securement of durability of the brake plates 47, 48 and securement of braking performance can be satisfied.