An automatic analyzer has a washing tank providing water for washing a reagent or sample probe is. The washing water from a washing nozzle spreads from a throttle portion and is divided into right and left flows after colliding with a vent plate provided in an overflow portion of the washing tank. The washing water flows between the vent plate and the inner wall of the overflow portion, and the flows join behind the vent plate. After joining, the washing water flows downward along the vent plate and the inner wall of the overflow portion and then is drained. Because a space is created between the washing water which has collided with the vent plate and the washing water joined behind the vent plate, the washing water is prevented from completely covering the overflow portion, and the airflow can be secured during the drainage.