A nozzle combining structure for a hot runner system for an injection molding machine comprising a manifold having a resin injecting hole extending therethrough in its inner portion through which a resin flows, the manifold having a nozzle seat groove formed in an outer peripheral surface of an end of the resin injecting hole, a nozzle being seated in the groove, and the nozzle combining structure further comprises: a nozzle having a piston guide bushing formed at its upper end inserted into the nozzle seat groove of the manifold, for dispensing the resin injected from the resin injecting hole of the manifold through the piston guide bushing into a mold; a manifold guide bushing for surrounding and threadedly engaging with an upper portion of the piston guide bushing of the nozzle, the upper end of the manifold guide bushing having a diameter smaller than that of the nozzle seat groove of the manifold, enough to accommodate corresponding volume change caused by the thermal expansion of the manifold, so that the upper end of the manifold guide bushing is inserted into the nozzle seat groove of the manifold; and a fastening bushing for receiving an outer surface of the manifold guide bushing and threadedly engaging with an inner circumferential surface defining the nozzle seat groove of the manifold so that the manifold guide bushing is fastened by the fastening bushing to the inner circumferential surface defining the nozzle seat groove of the manifold, the manifold guide bushing including a flange with an expanded diameter at a distal end of the manifold guide bushing for surrounding and threadedly engaging with the piston guide bushing at the upper end of the nozzle and the flange is locked on an upper surface of the fastening bushing threadedly mated with the inner circumferential surface defining the nozzle seat groove of the manifold and the inner diameter of the fastening bushing for threadedly mating with the inner circumferential surface defining the nozzle seat groove of the manifold is larger than the outer diameter of the manifold guide bushing enough to receive the volume change caused by thermal expansion of the manifold thereby guiding the manifold guide bushing on thermal expansion of the manifold. a nozzle having a piston guide bushing formed at its upper end inserted into the nozzle seat groove of the manifold, for dispensing the resin injected from the resin injecting hole of the manifold through the piston guide bushing into a mold; a manifold guide bushing for surrounding and threadedly engaging with an upper portion of the piston guide bushing of the nozzle, the upper end of the manifold guide bushing having a diameter smaller than that of the nozzle seat groove of the manifold, enough to accommodate corresponding volume change caused by the thermal expansion of the manifold, so that the upper end of the manifold guide bushing is inserted into the nozzle seat groove of the manifold; and a fastening bushing for receiving an outer surface of the manifold guide bushing and threadedly engaging with an inner circumferential surface defining the nozzle seat groove of the manifold so that the manifold guide bushing is fastened by the fastening bushing to the inner circumferential surface defining the nozzle seat groove of the manifold, the manifold guide bushing including a flange with an expanded diameter at a distal end of the manifold guide bushing for surrounding and threadedly engaging with the piston guide bushing at the upper end of the nozzle and the flange is locked on an upper surface of the fastening bushing threadedly mated with the inner circumferential surface defining the nozzle seat groove of the manifold and the inner diameter of the fastening bushing for threadedly mating with the inner circumferential surface defining the nozzle seat groove of the manifold is larger than the outer diameter of the manifold guide bushing enough to receive the volume change caused by thermal expansion of the manifold thereby guiding the manifold guide bushing on thermal expansion of the manifold.