To efficiently remove sand stagnated in the bush of pile yarns in a foot wiping mat without damaging the mat.

In a method for washing a mat wherein a large number of pile yarns 3 are implanted in a base fabric, at first, water is supplied to a pile yarn arranged surface. Next, the pile yarns 3 are frozen in a moisture sufficiently containing state by a freezing means such as liquid nitrogen. Solid foreign matter such as sand is removed from the bush of the frozen pile yarns 3 along with ice by a mechanical removing means such as a rotary blade 24. In a moisture supply process, the mat is immersed in a water tank and high pressure water is sprayed on the pile yarns in the water tank to also preliminarily wash the pile yarns.