To provide a phenol resin foam which is easy to manufacture, free from problems with storage stability and has a reduced water-absorption by subjecting the foam to water-repellent treatment.

In the manufacturing process of the phenol resin comprising adding a foam stabilizer, foaming agent, acidic curing agent and modifier to a phenol resin followed by foaming and curing, 0.2-7 pts.wt. of a polyalkyl hydrogen siloxane expressed by general formula [1] (R is a 1-18C alkyl group) as the modifier is added to 100 pts.wt. of the phenol resin.