PROBLEM TO BE SOLVED: To provide a drain channel structure of a tank, which prevents entrainment of air from an overflow drain channel during draining and can eliminate gurgling sound during draining.SOLUTION: A drain channel structure has a trap 3a on the downstream side of a junction part P of a main drain channel 1 having an upstream side opening 1a in a bottom 4a of the tank 4 and an overflow drain channel 2 having an upstream side opening 2a in the vicinity of an upper edge of the tank 4. A downstream side opening 1d of the main drain channel 1 in the junction part P and a downstream side opening 2e of the overflow drain channel 2 are arranged to be faced to each other, drain is made to flow into the overflow drain channel 2 from the main drain channel 1 during draining, the overflow drain channel 2 is blocked, and the entrainment of air from the overflow drain channel 2 is prevented.
