To provide a ground improving material which can be easily laid on an existing ground while a seismic force can be reduced at low costs without changing a convenient detached house construction method.

The ground improving material, which is a material for improving a subsurface layer when constructing a building, is composed of a mat 120 to be laid totally or partly on the surface layer corresponding to the building for damping a seismic force of the ground surface. The mat when in use has a dominant frequency f which is indicated as f=(1/2π)(K g τ/W)1/2(provided that W is a building load (kN), K is a spring constant (kN/cm) of the mat receiving the load W, g is a gravity acceleration (cm/s2), and τ is a response magnification) and also is between 0.5-2.5 Hz.

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