A method of manufacturing a bathtub includes a first operation of forming an external structure layer forming an external appearance of the bathtub; a second operation of forming a mold-release agent layer having a bathtub forming surface corresponding to an internal shape of the bathtub and a flange extending from a circumference of the bathtub forming surface to the outside, on an external surface of an internal mold; and a third operation of forming a semi-rigid foam layer having a first unfoaming painted layer, a soft foam layer, and a second unfoaming painted layer in sequence such that the soft foam layer is sandwiched between the first unfoaming painted layer and the second unfoaming painted layer. The semi-rigid foam layer is formed between the external structure layer and the mold-release agent layer by being injected in a space between the external structure layer and the mold-release agent layer.