Provided is a modified diene based polymer in which a diene based polymer is graft modified with a polyether having a mercapto group attached via diurethane bonding. Formation of diurethane bonding is performed by allowing sequential reactions of two species of hydroxyl group-containing compounds with an diisocyanate compound. By adding the modified diene based polymer to a silica-containing rubber composition for automobile pneumatic tires, the dispersibility of silica compounded in the rubber composition can be improved, and a property inherent in silica which simultaneously allows a reduction of rolling resistance and a stability on wet road surfaces can be sufficiently effected.