A microbubble generator (20) includes an outer cylinder (201), bulkhead (202) and an inner cylinder (203) extending in the downstream direction from the bulkhead (202). A plurality of liquid passage holes (202a) is formed in the central portion of the bulkhead (202), and a circumferential flange (203a) is formed at the downstream end of the inner cylinder (203). A disk (204) is disposed adjacent to the rear edge of the inner cylinder (203). The disk (204) and circumferential flange (203a) define a restriction passage (17) in conjunction with each other. The circumferential flange (203a) forming one of the wall surfaces of the restriction passage (17) has a recess (206) formed therein.