To provide a construction method of a mat foundation for a wooden house excellent in strength preventing increase of under floor temperature to prevent corrosion of a sill or the like.

After the ground surface 4 of the whole site where a wooden house is erected is tamped, a crushed stone layer 6 is formed in the whole area of the ground face, a separation sheet 7 is laid so as to cover the whole surface of the crushed stone layer 6. Reinforcements 16 are assembled on the separation sheet 7 to temporarily erect a form and then, concrete is poured therein. By constructing the bottom 2 forming the mat foundation 1 and the rising part 3, water permeating from the underground through the crushed stone layer 6 is cut off to prevent water from infiltrating in the mat foundation 1. By providing a slope face 14 at the outer peripheral edge of the bottom 2, water can be prevented from infiltrating from the boundary part of the rising part 3 into the inside of the building.