Masonry units (10) are integrally assembled by tightening forces of bolts (60) and nuts (70), in a condition that interlayer metal plates (50) are interposed between the masonry units. A basic form (11) of the masonry unit is contained in a mold for forming the masonry unit. A space (98) for fluidic covering material, which is chargeable with the covering material, is defined between upper and lower faces of the form (11), and the fluidic covering material is charged in the space. The upper and lower faces of the masonry unit are covered with the solidified covering material (12). According to the present invention, high dimensional accuracy of the masonry unit can be ensured, and efficiency of the production process of the masonry units is promoted to improve productivity of the masonry units.