A method of inserting into a small-sized accommodating case an elastic body larger than the case by regulating the amounts the body is compressed in vertical and horizontal directions such that the elastic body adheres closely to the inner surface of the case without forming gaps therebetween and a distribution of dense and less-dense parts is generated in the inserted elastic body. First, the elastic body is compressed by pushing two sets of opposite side surfaces thereof by using two parallel clamping hands, respectively. Then, the compressed elastic body is inserted in the case. Subsequently, the elastic body is further compressed in the direction of the insertion. Thereby, the distribution of dense and less-dense parts can be generated in the elastic body without forming wrinkles therein and gaps between the elastic body and the inner surfaces of the ink tank. Consequently, ink supply capability can be raised and printing quality can be improved.