A drain board installing device without an anchor plate is disclosed. A drain board temporary clamp device 146 comprises a protrusion body 321 provided at the center part of an inner wall of the through hole below the drain casing; a plate shaped arm 333 provided with a slit to allow the arm to ascend and descend at a predetermined length along a longitudinal direction of the drain casing within the through hole of the drain casing under the guide of the protrusion; and a T shaped rod 335 fixed to a lower part of the plate shaped arm, wherein, when the plate shaped arm 333 and the T shaped rod 335 descend toward the lower end of the drain casing within the maximum range by means of their load, a lower end part 333a of the plate shaped arm descends to the part where a lower end part 204a of the drain casing is located, and when the plate shaped arm 333 and the T shaped rod 335 move toward the upper end of the drain casing within the maximum range by means of their load, a T shaped end part 335a of the rod is caught in a slit 154 formed in the lower end part of the drain casing.