Mice were housed in a cage with a T-maze for 7-9 h a day. A watering place was equipped at one end of the maze. At the other end of the maze, the right and left arms each have two exits, one of which leads to the home cage where food is placed, and the other of which leads to the watering place via a bypass. The exit leading to the home cage in either the right or left was alternately closed every two hours. One-way swinging doors were inserted at the entrance to each arm and between each bypass and the watering place. The mice housed in this apparatus acquired the alternation task at 5 s delay on the 6th day. As the delay became longer (5-150 s), correct response rate showed gradual decrease. Scopolamine (0.1-0.4 mg/kg) did not decrease correct response rate at 5 s delay, but did at longer delay (30 s and 60 s). These results suggest that this method is useful in estimating working memory of mice.