To provide a precast foundation and a construction method of the precast foundation allowing the construction of a bathroom unit and an underfloor storage unit in a well settled manner without impairing the mutual joint strength of foundation members.

Auxiliary reinforcements 210 are provided at the lower parts near the end parts of bodies 21, 21B of foundation members 2A, 2B so as to project inside the precast foundation 1, and the auxiliary reinforcements 210 are embedded with embedding members 5. The foundation members 2A, 2B are thereby jointed to each other. Consequently, there is no need to fill concrete slurry into a joint recessed part for connection. As a result, there is no need to bulge the side faces of the foundation members 2A, 2B.