PURPOSE: To obtan a thermosetting resin composition which is excellent in strength at high temperatures and useful as casting, varnish, laminate, and FRP materials, by mixing hexamethylenetetramine with a condensation product of phenol and a specified dialdehyde.

CONSTITUTION: A condensation product is obtained by the reaction of a phenol (i), e.g., phenol, cresol, etc., and a dialdehyde (ii) selected from glyoxal, glutaraldehyde, and terephthalaldehyde in a proportion of 1:0.1W1.0 by molal ratio at 40W 200°C in the presence of an acid catalyst, e.g., HCl, etc. And 100pts.wt. said condensation product (a) is mixed with 5W30pts.wt. hexatetramine (b) to obtain an objective resin composition. The resin composition is further mixed with a filler such as glass fiber, etc., as needed, and then heated to 80W200°C to form a hardened product.