PURPOSE: To change the bounding coefficient of an artificial turf by making the elastic modulus of a mat adjustable.

CONSTITUTION: Many tubes 12 made of flexible synthetic resin such as vinyl chloride are tangled to the preset thickness (t) to form a mat 10. A turf-like surface material 14 is fixed on the mat 10. Water is fed and sealed into the tubes 12 via a pressure governor 18 into the pressurized state. The water pressure sealed in the tubes 12 is optionally adjusted by the pressure governor 18, and the elastic modulus, i.e., bounding coefficient, of the mat 10 is changed.