This paper gives a general description of a new ship model experiment tank, “Mitaka No. 3 Ship Model Experiment Tank”, built at the Ship Research Institute. The principal dimensions of the tank are 150 m×7. 5 m×3. 5 m. The main features of the tank are as follows : 1. towing carriage of box girders convenient to perform various kinds of experiments, not only on ship models but also on ocean structures. 2. strong tank wall and special apparatus, which enable the accurate tests in shallow water. 3. sub-carriage carrying a small wind tunnel to perform the tests in waves with wind. 4. wave maker of plunger type whose vertical position can be varied corresponding to the water level. 5. easy control of the towing carriage any one can drive.