Abstract: The present invention relates to a throw-in type colorimeter having a defoaming mechanism comprising a double-cylinder whereby it is possible to very easily detect the optical density of the solution to be examined, and more particularly to a thrown-in type colorimeter equipped with a defoaming device in which a downward flow is created in a cylinder by means of the liquid level column difference in the inlet of the cylinder to let the bubbles in the solution escape upwardly therefrom and also equipped with an upward solution flow passage to provide a communicating passage between the outer and inner cylinders to form a down flow in the outer cylinder and drive the remaining bubbles out of the solution in the outer cylinder by the twice reversed flow while removing the bubbles residing in the measuring chamber by the agitated solution flow.