Abstract: A fitting device includes a connection terminal, a tubular housing that covers an outer circumference of the connection terminal, a rubber ring attached to an outer circumferential surface of the housing, and a retainer that prevents the rubber ring from being dislodged. The retainer includes a tubular main body portion, and a plurality of arm portions extending from the main body portion in a front-rear direction. Each arm portion includes an extension portion extending from the main body portion toward the rubber ring in the front-rear direction, a first projection protruding from the extension portion toward an inner circumferential surface, and a second projection protruding from the extension portion toward the outer circumferential surface of the housing. An amount of insertion of the second projection into the recess is larger than a gap between a protruding end of the first projection and the inner circumferential surface.