To provide a septic tank in which a thick wall part and a thin wall part are not generated to a short pipe.

In this septic tank, a septic tank body wall 1 inclined in relation to a vertical plane P is provided with a through hole 11, and the other end of a short pipe 2 with a flange 21 provided at one end is inserted in the through hole 11 from the inside of the body wall 1. The flange 21 is provided along a plane inclined in the same degree as the body wall 1 in relation to the vertical plane P right-angled to the center axis of the short pipe 2, and a nut 3 is screwed in from the other end of the short pipe 2. One face of the flange 21 is thereby spliced to the inner surface of the body wall 1, and the other end of the short pipe 2 is protruded almost horizontally to the outside of the body wall 1 so as to form an outflow pipe connecting part. In this case, a tubular spacer 4 is inserted between the outer surface of the body wall 1 and the nut 3. One end face of the tubular spacer 4 is inclined in the same degree as the body wall 1 in relation to the vertical plarie of the center axis of the tubular spacer 4, and one end face of the spacer 4 is spjtced to the outer surface of the body wall 1.