PURPOSE: To provide a flexible resin compsn. which is suitable for use in insulating varnish, insulating paint, etc. and as a casting material and has excellent flexibility, adhesion and crack resistance, by blending petroleum resin with an arom. hydrocarbon/formaldehyde resin.

CONSTITUTION: Petroleum resin is mixed with an arom. hydrocarbon/formaldehyde resin such as a xylene/formaldehyde resin, a phenol/formaldehyde resin, a cresol/formaldehyde resin or a xylene/formaldehyde resin. The mixture is reacted by heating of at a temp. of pref. 80°C or higher to obtain the desired flexible resin compsn. As the petroleum resin, there are used those obtd. by subjecting a compd. boiling at 20W280°C such as butene, pentadiene, isoprene, styrene or indene obtd. from petroleum to steam cracking or vapor-phase thermal cracking, and then polymerizing the compd.