To provide a wet disc brake that excellently maintains braking capabilities by improving cooling capabilities with respect to rotary discs and brake friction plates by returning heated cooling oil while cooling it without using the rotary discs and the brake friction plates.

The wet disc brake 200 includes a brake case 203 formed with rotary discs 202, brake friction plates 207, and a storage part 204 for storing cooling oil. The brake case 203 is composed of a first case 205 and a second case 206. A large-diameter part 205a is formed inside the first case 205 while each cutout part 205c is radially formed around the large-diameter part 205a. A gap S is formed between the bottom part 205d of the large-diameter part 205a and a plate pressure 210 while communicating with each cutout part 205c. Each guide groove 206b is radially formed in the side face 206a of the second case 206.