To solve problems that Shirasu which is volcanic ejecta is not effectively used, various kinds of slag is dealt with as industrial waste and these are recently started to be utilized for a base course material or the like as a construction material but a lot of them are unused.

Safe and stable coarse aggregate of the slag is obtained by coating the surface of slag granular material with resin cement or the like and fine particulate Shirasu. The coarse aggregate, the volcanic ejecta (Shirasu) having water holding power as fine aggregate and cement as a binder are used and stirred, mixed and formed to obtain a porous concrete product having water holding property and high water permeability. The problems are easily solved in this way. Goods friendly to natural environment are developed using the low cost materials and the waste slag and the volcanic ejecta (Shirasu) are effectively utilized.