To provide forms for embedding a septic tank suitable for holding a hole excavated for installing the septic tank so as not to collapse and being fitted for safely and smoothly conducting the installation works of the septic tank.

Short-sized connecting plates 15 and long-sized connecting plates 17 are clamped to four corner frame members 1 by clamping bolts and assembled into a casing shape, supports 20 are brought to a type in which the supports are fitted movably into the guide holes of each corner frame member 1 respectively, and the forms are mounted into a hole excavated by using a small-sizes crane or the like under the state in which the supports 20 are extended to lower sections from sections assembled in the casing shape and fixed. A large number of sheet piles are inserted into openings (s) in a fixed size formed inside the connecting plates 15, 17 for the forms positioned in upper sections, and these sheet piles are driven into ground surface.