NOVELTY - A tortoise-like septic tank comprises an inlet, a septic tank and an outlet. The inlet on one transverse surface of septic tank is made of a rectangular inclined bottom; and the outlet on the opposite surface is made of a cylindrical flat bottom. The septic tank is a rectangular trapezoid, with its bottom made of semicircular inclined bottom inclined to outlet from the inlet. The tank surface is a tortoise back type.    USE - Tortoise-like septic tank used for collecting human waste and producing biological energy methane.    ADVANTAGE - The septic tank benefits the people and protects the environment, meets the requirements of creating a resource saving and environment protecting harmonious society, and can create economic benefits.    DETAILED DESCRIPTION - A tortoise-like septic tank comprises an inlet, a septic tank and an outlet. The inlet on one transverse surface of septic tank is made of a rectangular inclined bottom; and the outlet on the opposite surface is made of a cylindrical flat bottom. The septic tank is a rectangular trapezoid, with its bottom made of semicircular inclined bottom inclined to outlet from the inlet. The tank surface is a tortoise back type. The septic tank is built by six-step method as follows: (1) the tank is a rectangular trapezoid with the same width and depth, and is built in a certain depth under the ground level; (2) the volume cubic number of tank is divided into three parts; and the 1/3 number is added to calculate the floor area; (3) the pitch of the bottom of the tank is 1/4 depth of the tank; and the bottom is downwards semi-circularly inclined to the outlet from the inlet; (4) the tortoise-like construction line of tank surface is within 3/4 of the depth of the tank; the 3/4 depth number is divided into four parts to draw the line on the tank surface; (5) the construction line of the tank surface orthogonal to the side surface is the arc line which is drawn from the angle point of the horizontal line in the 1/4 depth part place of four corners of the tank to the 3/4 height part place of the center point of the surface; and (6) the construction line of the tank surface transverse to the side surface is the arc line which is drawn from the angle point of the horizontal line in the 1/4 height part place of the tank corners to the 2/4 height part place of the center point of the surface.