An object positioning and fixing device (1; 10) that positions and fixes a target object (3; 3B; 3C; 3D) on a base member (2; 2A; 2B; 2C; 2D) in a horizontal direction and in a vertical direction, wherein: said base member comprises a reference seating surface for positioning in the vertical direction, and said target object comprises a contacting surface that can contact against said reference seating surface; one of said base member and said target object comprises a fitting hole that is formed to open to said reference seating surface or to said contacting surface, and a reference member (10, 20) that is fixed in said fitting hole; the other of said base member and said target object comprises an installation hole that is formed to open to said contacting surface or to said reference seating surface at a position corresponding to said reference member, and an annular engagement member (30, 40) that is fixed in said installation hole; and said reference member comprises a fitting cylindrical portion that is fitted into and fixed in said fitting hole, an engagement projection (13, 23) that is formed integrally with said fitting cylindrical portion so as to have a larger external diameter than said fitting cylindrical portion and that projects from said reference seating surface or said contacting surface toward said the other, a tapered engagement surface (10a, 20a) that is formed on at least a portion of an external peripheral surface of said engagement projection and whose diameter becomes smaller towards said the other, and a base end contacting surface that is formed on a base end of said fitting cylindrical portion side of said engagement projection and that contacts against said reference surface or said contacting surface; characterized in that said annular engagement member comprises a tapered contacting surface (30a, 40a) that, when said contacting surface is contacted against said reference seating surface, can engage in a tightly contacting state with said tapered engagement surface of said engagement projection, via elastic deformation of said annular engagement member in the radially outward direction. said base member comprises a reference seating surface for positioning in the vertical direction, and said target object comprises a contacting surface that can contact against said reference seating surface; one of said base member and said target object comprises a fitting hole that is formed to open to said reference seating surface or to said contacting surface, and a reference member (10, 20) that is fixed in said fitting hole; the other of said base member and said target object comprises an installation hole that is formed to open to said contacting surface or to said reference seating surface at a position corresponding to said reference member, and an annular engagement member (30, 40) that is fixed in said installation hole; and said reference member comprises a fitting cylindrical portion that is fitted into and fixed in said fitting hole, an engagement projection (13, 23) that is formed integrally with said fitting cylindrical portion so as to have a larger external diameter than said fitting cylindrical portion and that projects from said reference seating surface or said contacting surface toward said the other, a tapered engagement surface (10a, 20a) that is formed on at least a portion of an external peripheral surface of said engagement projection and whose diameter becomes smaller towards said the other, and a base end contacting surface that is formed on a base end of said fitting cylindrical portion side of said engagement projection and that contacts against said reference surface or said contacting surface; characterized in that said annular engagement member comprises a tapered contacting surface (30a, 40a) that, when said contacting surface is contacted against said reference seating surface, can engage in a tightly contacting state with said tapered engagement surface of said engagement projection, via elastic deformation of said annular engagement member in the radially outward direction.