PURPOSE: To provide the titled compsn. having excellent rigidity and resistance to heat and impact, consisting of a ring-opening norbornene polymer and a specified heat-resistant stabilizer.

CONSTITUTION: 0.05W15pts.wt. at least one heat-resistant stabilizer selected from compds. of formulas III to V (wherein R1 is methyl, t-butyl; t-Bu is t-butyl; R6 is H, a 1W3C alkyl) and compds. of formulas VI to VIII (wherein m, n are each 12W 18) is blended with 100pts.wt. ring-opening norbornene polymer which has an intrinsic viscosity of 0.3W4.0 and is composed of 30W100wt% structural unit of formula I (wherein R1 is H, alkyl, phenyl; R2 is H, alkyl) and 70W0wt% structural unit of formula II (wherein R3 is alkyl) and in which the content of a trans double bond is 40% or above.