A diffusion plate includes a plate member facing an opening end of a supply pipe with a thickness direction thereof substantially parallel with a supply direction of a supply fluid. The plate member is rotatable around a rotation axis substantially parallel with the thickness direction and has a diffusion hole, penetrating in the thickness direction at a position other than a rotation center of the plate member. The diffusion hole has a wall surface defined at a rear side in a rotation direction and has a first wall surface end defined in a facing surface facing the supply pipe, at a rearmost in the rotation direction and a second wall surface end defined in a non-facing surface at a rearmost in the rotation direction. The wall surface is inclined with the first wall surface end being at a front side of the second wall surface end in the rotation direction.
