With the increase in the application of CLSM using coal ash, we performed a basic research on CLSM material, laying focus on the correlation between compressive strength and unit weight of lightweight foamed CLSM. The unconfined compression strength is a criterion for the judgment of the possibility of re-excavation and an important factor determining the economy, efficiency, and excavation character. However, to know the quantitative compression strength value takes a certain amount of time, because the applicability of unconfined compression strength of CLSM is judged by the standard of 28days. Therefore, in this study the relation between compressive strength and unit weight (foam slurry unit weight, apparent unit weight) is analyzed focusing on lightweight foamed CLSM. We also suggested a formula which can easily predict the 28-day compressive strength only using unit weight value without the need to cure the slurry for 28 days.