PURPOSE: To completely prevent leakage from the opposite surfaces of a bottom plate and a side frame with a low priced material for a leakage prevention method between the side frame and bottom plate of a water storage tank.

CONSTITUTION: Horizontal recessed grooves 7 and 8 opposed to a bottom plate 6 fastened with an angle 13 and inserts 10 and 12 extended over the whole area of the circumference of a side frame 1a of a water storage tank buried in the ground is formed in the side frame 1a. Force-fit of an elastic backup material 11 is made between the opposite surfaces of the lower side of the recessed groove 7, water expansive rubber or solutional sealing compound 14 is injected in the recessed groove 7 of the upper part of the back-up material 11 to fill, and the upper part thereof is filled with non-contraction mortar 15.