To obtain a lightweight aggregate having a high strength and a low water absorption by adding a foaming agent and SiC to an expandable clay as a raw material, and then calcining the mixture.

The expandable clay includes Tienchin clay, loess and Hwang Ho deposits. The expandable clay and SiC are homogeneously and finely ground and mixed and then compactly molded. When the molded product is liquidized at a calcination temperature of 1,100-1,200°C, the SiC is wetted and reacted with the melted liquid to generate a gas. When the SiC is covered with uniform glass or the like, the generated gas is closed in the sealed spaces to produce independent spherical pores. When fine clay is used, a finely grinding process can be omitted. For example, the employment of the loess permits the omit of the finely grinding process. The lightweight aggregate is, for example, obtained by adding 0.2% of SiC to loess, milling the mixture with a pot mill for 6 hr to prepare the milled product having an average particle diameter of 3 μm, dehydrating and drying the milled product, forming the dried product into about 10 mm pellets, calcining the pellets at 900°C in an electric oven and then foaming the pellets at 1,160°C.