To provide an opening structure of a building excelling in the degree of design freedom by forming a structure easily changed in the position of a window frame to an external wall to make adjustable the length of an opening side piece, extending in the direction of the window frame, of an external facing material for an opening.

The opening structure of the building is provided with the opening frame 18 surrounding a window opening 3 opened in the external wall 1; opening frame backing 5 formed in frame shape with an outer peripheral dimension movable in an indoor-outdoor direction along the opening frame 18 and with a width dimension in the indoor-outdoor direction storable within the opening frame 18, and disposed on the indoor side rather than the outdoor side end of the opening frame 18 and mounted to the opening frame 18; the window frame 21 mounted to the inner periphery of the opening frame backing 5 in a position on the indoor RM side from the surface of the external wall 1; and the opening external facing material 6 mounted to the peripheral edge of the opening 3 continuously with the external facing material 13 covering the external wall 1, and having an opening frame side piece 6b covering a portion, located on the outdoor side rather than the window frame 21, of the opening frame 18.