Resin casting mold, molded resin product, optical element, optical scanning device, and image forming apparatus

A resin casting mold includes a cavity having a predetermined shape and is used for producing a molded resin product by transferring the shape of a surface on the cavity side of a mold member forming the cavity to a resin material. The surface on the cavity side of the mold member includes first and second surfaces facing each other in a first direction and each of the first and second surfaces have a predetermined surface shape, the mold member includes an insert member which forms a part of a third surface different from any one of the first and second surfaces of the surface on the cavity side and which is slidable in a second direction perpendicular to the first direction, at least one cross-sectional shape of the cavity satisfies a relation h < A < H where A is the length of the insert member in the first direction, h is a spacing in the first direction between a portion of the first surface closest to the insert member and a portion of the second surface closet to the insert member, and H is the length in the first direction on the third surface, and the insert member is disposed so as to include a region having the spacing h in the first direction.