To solve a problem that a reverse flow preventive function is possibly not sustained for an ink container having a large diameter, when reverse flow preventive fluid is used in the ink tank for an ink jet printer having a plurality of ink tank parts integrally where each ink tank part does not have an identical shape, the assembling work becomes troublesome and expensive when floats are arranged because a float fitting to each ink tank part must be inserted therein while discriminating, and failure of a product is apprehended when the float is arranged erroneously.

Each ink tank part is arranged with the float dipped at least partially into a layer of the reverse flow preventive fluid and provided with an identification mark, e.g. an ink color mark, for the corresponding ink tank.