PURPOSE: To cure even a large-size products by steam in a labor-saving manner by a method wherein a heat-insulating wall is provided outside a diaphragm at a predetermined interval, and a steam curing space is formed between the diaphragm and the heat-insulating wall.

CONSTITUTION: A heat-insulating wall 3 is provided outside a diaphragm 2 of an L-shaped retaining wall block form 1 at a predetermined interval. A steam curing space 4 is formed between the diaphragm 2 and the heat-insulating wall 3. The heat-insulating wall 3 is formed to be slightly higher than the diaphragm 2. Thereon, a cover 5 is placed, whereby a steam curing space 4 is formed also between the cover 5 and a concrete surface 14. Steam is supplied to the curing space 4 from a supply opening 8. All the steam curing space 4 is filled with steam. Casted concrete is easily cured by steam. Therefore, there is no need for covering the form 1 with a sheet or the like, and a concrete secondary product can be efficiently obtained.