The present invention relates to ruthenium complex compound wherein a heteroatom-containing carbene compound having a six-membered ring structure is bonded to ruthenium and a process for the preparation thereof; a process for producing ring-opened cyclic olefin polymer by subjecting a cyclic olefin to ring-opening metathesis polymerization in the presence of such a ruthenium complex compound; and a process for producing hydrogenated product of ring-opened cyclic olefin polymers by hydrogenating the carbon-carbon double bonds of the above ring-opened polymers. The invention gives hydrogenated product of ring-opened cyclic olefin polymers having physical properties different from those of hydrogenated product of ring-opened polymers of the prior art. Particularly, the use of dicyclopentadiene can give a hydrogenated product of ring-opened cyclic olefin polymer excellent in heat resistance, and this product can be advantageously used as parts or formed products for medical appliances or electrical components.