To provide an assured cooling performance even at a final passage part of a water cooling jacket of a diffusion cylinder, while minimizing a pressure loss.

The water cooling jackets 5a, 5b are formed between an inner cylinder 2 and an outer cylinder 3 of the diffusion cylinder 1. Cooling water is supplied from an inlet header 6 via a cutout K1 to the water jackets 5a, 5b for circulation and discharged via cutouts K2, K2 to an outlet header 7. Cutouts K2, K3 formed at the outlet of the outer cylinder 3 are adapted such that the branching ratio is to be 0.4 or lower, or three or more cutouts K2 are to be provided for the cooling jacket 5a and three or more cutouts K3 are to be provided for the cooling jacket 5b. This reduces cooling water stagnation in the water cooling jackets 5a, 5b, enabling assured cooling performance along with minimized pressure loss.