PURPOSE: To obtain the titled resin easily, without using the washing process nor causing the corrosion of the apparatus, by adding a solid catalyst supporting phosphorus pentoxide to a feed oil containing a polymerizable component, and polymerizing the oil.

CONSTITUTION: 100pts.wt. of a feed oil containing a polymerizable component, i.e. the gas oil produced by the coal carbonization or the 130W200°C fraction produced by the distillation of coal tar, is added with 0.1W20pts.wt., preferably 1W10pts.wt. of a solid catalyst obtained by slurrying phosphorus pentoxide with a carrier such as diatomaceous earth, and kneading and calcining the slurry. The objective liquid petroleum resin or liquid coumarone resin having a viscosity of 1 poise (25°C)W30 poise (50°C) is produced by polymerizing the above mixture.