To provide a ready-to-assemble bathtub which introduces an engagement protrusion which is circular as viewed from above and an engagement groove on the join of each side plate to form a peripheral wall, and varies little in quality while being able to share the side plate, regardless of the bottom size and shape of the bathtub.

The ready-to-assemble bathtub keeps a wooden peripheral wall which puts two or more perpendicular side plates in a row attached to the outer circumference of a bottom wall with a loop shaped contour and makes an angle of junction between respective side plates an obtuse angle laterally outward. An engagement protrusion 10 which has a circular shape as viewed from above is arranged longitudinally at one end of two side plates 8 to face one another at the join so that the angle of junction Ac can be adjusted along the contour of the bottom wall 2, and an engagement groove 14 which has a cross-sectionally circular receiving surface 20c which receives the engagement protrusion 10 at the other end rotatably is arranged longitudinally respectively. Furthermore, a mathematical formula 2r<d is approved when the radius of curvature of the engagement protrusion 10 and the engagement groove 14 is r and the board thickness of the side plate 8 is d.