To provide a foundation construction method which constructs a foundation on a soft ground by a caisson method which can easily set the foundation at a predetermined location without subsidence of a caisson.

There is provided a foundation construction method for constructing a foundation on a soft ground G by using a caisson method. The ground G in a set area 3 is excavated, and sheet piles 4 are placed around an intended area 5 in the set area 3. Then, the ground G in the intended area 5 is excavated to a depth at which the foundation is buried, and a caisson 6 is set in the intended area 5. The caisson 6 consists of a lower caisson 6b to be buried under a ground surface 1 after completion of the foundation, and an upper caisson 6a to be dissembled on the ground surface 1 after the completion of the foundation. Further, a filler 7 is placed on the lower caisson 6b, and concrete is placed on the upper portion of the lower caisson 6b and in the upper caisson 6a, to thereby construct a slab 8 and a base body 9. Thereafter, the upper caisson 6a is dissembled and taken away, and the intended area 5 and the set area 3 around the lower caisson 6b are back-filled.