This paper gives a general description of a new ship model experiment tank, “Mitaka No.2 Ship Model Experiment Tank”, built at the Ship Research Institute. The principal dimensions of the tank are 400m × 18m × 8 m with the breadth and depth largest in the world. Because of this huge size of the tank, it took four years to complete all the facilities. In this report are given design considerations and general descriptions of the construction of the tank, which includes the tank itself, the rails, the carriage, the trolley wires, the wave absorbers, the wavemaker, the measuring instruments, the data processor, the building to house facilities, and the workshop and the office. The main features of the experiment tank are as follows ; 1. large cross section of the tank. 2. deep and shallow trimming tanks with watertight doors. 3. four kinds of brake systems for the carriage. 4. air-conditioned room on a side part of the carriage. 5. accurate speed control of the carriage. 6. rigid trolley wires. 7. movable wave absorbers at the both sides of the tank wall. 8. measuring instruments of digital type with remote control system. 9. on-line data processing system.