PURPOSE: To enable fabrication of a sump pit with different capacities by connecting intermediate plates between the sides of inner and outer forms which constitute a sump pit.

CONSTITUTION: Four inner-form corner poles 3 each equipped with a flange 11 are placed upright at the four corners of an inner form 1 at predetermined intervals, and on each side of the inner form 1 between the corner poles 3, an intermediate plate 7 provided with grip angles 9 at both sides is connected to the corner poles 3 via bolt holes 13 and threaded members 15. In an outer form 2, outer-form corner poles 4 each equipped with an outer-form flange 12 are similarly placed upright to surround the inner form 1, and an intermediate plate 8 provided with grip angles 9 at both sides is connected between the corner poles 4 to form a sump pit. Further, the grip angle 9 has a small angle θthat satisfies the condition of 90°≤θ≤105°, so as to facilitate removal of the inner form 1 and the outer form 2 after concrete sets. Sump pits of various sizes can thus be fabricated simply through the preparation of various sizes of intermediate plates 7, 8.