To stably generate a low level radioactive ray and negative ions at all times.

Tatami floor members 2 in which radon ceramic rings 7 formed by baking a natural radium mineral in a ring shape are incorporated, heating members 3, and tatami mat surface members 4 are stacked on each other in this order. The stacked body is sewn at the tatami mat edges 13. The tatami floor members 2 incorporate the radon ceramic rings 7 of the generally same thickness as a hard foam plate in holes 6a drilled in the hard foam plate 3. A cushion material is stuck on the lower surface of the tatami mat on which a tatami is to be installed. The heating member is a film heating layer. The tatami surface member is a natural or synthetic rush formed by sticking a nonwoven fabric on the heating member side.