To make a construction work efficient by applying the constitution that lightweight mortar with many beads of granular cellular plastic contained in mortar is solidified and cured.

Water 10, cement 11, fine aggregate 12 such as sand, and expanded styrene beads 13 are respectively measured and fed to a mixer 14 so as to be kneaded. The kneaded lightweight mortar 17 is forcibly sent to a curing yard 16 on the operation of a Mono pump 15, and solidified and cured in the atmosphere until the strength thereof reaches a prescribed level. Then, the solidified lightweight mortar 17 is crushed by use of a giant breaker 18 and formed into gravel type adjusted for suitable particle size distribution. The gravel type lightweight mortar 17 so prepared is loaded into a bottom-hopper barge, and the bottom thereof is opened at a prescribed place for throwing the mortar 17 into the sea, thereby reclaiming or filling a weak subsoil in the ocean. Thus, the mortar 17 is very suitable for the reclamation and the filling of the weak subsoil and a large amount of the mortar 17 can be manufactured, thereby ensuring an efficient construction work.