PROBLEM TO BE SOLVED: To provide lightweight aggregate in which waste rock wool is utilized and which is produced at a low cost, and also to provide a production process of the lightweight aggregate. SOLUTION: This lightweight aggregate is produced by granulating powdery waste rock wool having a <=2 mm average fiber length into an almost spherical granulated material by using an inorganic hydraulic material and has a 85-98 wt.% powdery waste rock wool content. This production process comprises grinding waste rock wool into powdery waste rock wool having a <=2 mm average fiber length, mixing the powdery waste rock wool, an inorganic hydraulic material and water together to obtain a mixture, granulating the mixture into a granulated material and screening the granulated material to produce the objective aggregate having a 85-98 wt.% powdery waste rock wool content.
