To provide a method for manufacturing a water-retentive block by vacuum extrusion molding without comprising a firing step, which gives the water-retentive block satisfying both enough strength and water retentivity and free from craze or crack at the vacuum extrusion molding even when fly ash is added.

A mixture is obtained by blending a clay material such as quarry waste soil, a solidifying material mainly comprising cement, an aggregate material mainly comprising clinker ash or crushed coal ash-solidified stone, and fly ash, where the total weight of the clay material and fly ash blended is 30-39 wt.% to the total weight, the weight of the fly ash blended is ≤50 wt.% to the total weight of the clay material and fly ash, the weight of the solidifying material blended is 15-38 wt.% to the total weight, and the weight of the aggregate material blended is 32-46 wt.% to the total weight. This mixture is kneaded and vacuum extrusion molded while deaerated by vacuum suction and then dried without firing to form the block body.