To inexpensively prevent dew condensation in a basement without use of an air conditioner.

A foundation floor 4 of the basement, a sidewall 5 thereof, and a ceiling 6 thereof are molded from concrete; a hollow concrete block 10 is laid on the concrete foundation floor; and a concrete floor 11, which is installed via a heat insulating material 9 on the hollow concrete block, is provided. Firstly, a slight gap is provided between the hollow concrete block 10 and the sidewall; secondly, a gap is provided in such a manner that the adjacent blocks are isolated from each other; and thirdly, a cavity of the block is arranged in such a manner as to form a linear cavity (claim 1). Since the hollow concrete block 10 is equipped with a narrow gap, air constantly flows by changing atmospheric pressure by a difference in indoor temperature. This prevents the dew condensation from occurring in the basement 1 due to the retention of the air.