PURPOSE: To avoid the blocking drain ditch or drain pipe provided at the bottom of aperture with mortar by laying a lath metal net almost covering the aperture between inner and outer walls on a position lower than a stacking position in the stacking construction work of concrete blocks.

CONSTITUTION: The inner wall 4 is formed by stacking and joining the concrete blocks 3 from the upper edge of a groove 5 with the aid of a mortar 7, and then, when the third-stage concrete block 3 is stacked up, a bracket 8 having a U-shaped basal end is mounted to the third-stage concrete block 3 by fixing its one basal end side with mortar by insertion, and its tip portion is extended laterally in the direction of outer wall. The bracket 8 is laterally mounted in parallel with the position of a block-reinforcing vertical steel bar 9, and a lath metal net 10 almost covering the aperture between the inner and outer walls is laid on the upper end portion of plural brackets 8. Thus, mortar used for joining is inserted and fixed in the metal net 10, preventing its dropping to the bottom of the aperture.