PURPOSE: To install a bathtub at a target level, and improve the installability by a method wherein mortar is laid on the floor surface of a bathroom, and jigs are stood under the flange of a bathtub which is mounted on the mortar, and after pressing down the bathtub until the flange comes into contact with the jig, the jigs are removed.

CONSTITUTION: When a bathtub is installed, mortar is laid first at a location where the bathtub is installed. At the same time, three or more jigs 6 equipped with a main piece 7 and supporting pieces 8, 9 which are pivotally supported at both ends of the main piece 7 with hinge pins 8a, 9a are stacked. In this case, by turning a level- adjusting bolt 15, the upper surface of the jig 6 on the uppermost stage is set under a specified level. Then, the legs of the bathtub are placed on the mortar, and the bathtub is pressed down until the flange of the bathtub comes into contact with the upper surface of the jig 6. Then, after pulling out the jigs 6 in the middle stage, upper stage and lower stage in order to the side where a body washing space is planned to be made, an internal plate 10 is wrapped by an external plate 11 for each jig 6, and the supporting pieces 8, 9 are folded so as to be placed on the top of the main piece 7 to remove the jigs 6.