To attach a mounting base body for louvers, which is fitted onto a sleeve embedded in a concrete wall body, completely and watertightly in an easy way.

Where a mounting base body 3 for louvers is fitted onto the open end part 2b of a sleeve embedded in a concrete wall body, a rubber O-ring 9 is fitted at the outer periphery of the fitting tubular part 3a of the mounting base body 3. The O-ring 9 is brought into pressure contact with the fitting tubular part 3a and the inner periphery of the sleeve so as to be crushed between them to close a clearance 11 generated between them for water cutting- off.