The present invention provides a solid phosphoric acid catalyst which attains high activity and dimer selectivity in olefin dimerization reaction and an efficient method for dimerization of olefin using the same. The solid phosphoric acid catalyst comprises phosphoric acid supported on a siliceous carrier, the proportion of orthophosphoric acid in the phosphoric acid supported being 60 mol % or more in terms of phosphorus atom amount. The solid phosphoric acid catalyst is prepared by bringing a phosphoric acid aqueous solution into contact with the siliceous carrier followed by drying, the preparation step being carried out at a temperature lower than 100.degree. C. Also provided is a method for bringing an olefin-containing raw material into contact with the catalyst. In this method, the olefin-containing raw material contains water in an amount of 10 to 1000 mass ppm, the olefin-containing raw material is brought into contact with the solid phosphoric acid catalyst in a liquid phase, and the olefin is a monoolefin having 3 to 7 carbon atoms.