A wall-mounted type toilet fixing device includes a flush water tank device that is hidden in an area behind a wall, and a fixing unit, the fixing unit includes support portions, a toilet fixing portion, a pair of left and right legs, and a drain pipe connection, and the pair of legs are disposed outside from the drain pipe connection and inside from a side wall of a toilet body in elevation view, and are disposed laterally asymmetrically to each other with respect to a central axis passing through a center of the drain pipe connection and dividing the fixing unit equally in a left-right direction, in a state where the toilet body is fixed to the toilet fixing portion.

A flush toilet includes a toilet main body and a tank device for supplying flush water to the toilet main body, where the tank device includes a storage tank provided behind the toilet main body and above a floor surface, the storage tank includes a large tank section and a small tank section and has an asymmetrical shape in a front-back or left-right direction, the large tank section being a large-capacity side of the storage tank that is divided into two at a center in the left-right direction when seen in the front-back direction, the small tank section being a small-capacity side of the storage tank that is divided into two at the center in the left-right direction, and the storage tank or the toilet main body includes a deformation-preventing supporting member for preventing deformation of the storage tank by supporting the storage tank.