To obtain a panel for construction capable of accomplishing lightening without lowering strength by blending a hydraulic inorganic material with an ultra-lightweight aggregate and a silica admixture to give a mortar and hardening the mortar.

A double floor structure 10 is, for example, provided with a floor surface by standing a great number of struts 12 at fixed intervals on a floor slab 11 so as to execute wiring, piping, etc. under the floor of a building such as an office building and laying a great number of floor panels 13 made of concrete through the struts 12. The floor panels 13 are made by molding and hardening a cement mortar in plane approximately in square and backing a face opposing the floor slab 11 with a reinforcing iron plate 14. The cement mortar is obtained by blending 100 pts.wt. of a cement as a hydraulic inorganic material with 20-40wt.% of an ultra-lightweight aggregate and 5-30wt.% of a silica admixture. Perlite is adopted as the ultra-lightweight aggregate and fumed silica fume as the silica admixture in the cement mortar and the cement mortar has 1.0-1.5 specific gravity.