PROBLEM TO BE SOLVED: To provide an internal type filter device disposed in a water bath and enabled to filter water in the water bath quickly and efficiently over a wide range and to replace a filter medium easily.SOLUTION: A suction/drainage system B and an internal filter C are connected to the upper face and the side face of a pedestal A, respectively. The internal filter C has a thin plate-like filter medium 41 mounted retractably in a flat filter frame 30, and the flat outer face of the filter medium 41 has its substantially whole area exposed to the inside of a water bath V.