Method for reinforcing a hollow section by means of a plastic rod made from a hardened fibre-reinforced plastic extending on the inside of the hollow section and longitudinally thereof, • wherein a rod blank of the plastic rod is made with not yet hardened fibre-reinforced plastic and with an oversized blank cross-section, which extends in the vertical projection on the free hollow section cross-section beyond the free hollow section cross-section and which is elastically compressible and elastically expandable after the elastic compression, • wherein the oversized blank cross-section of the rod blank is elastically compressed outside the hollow section to become a compressed blank cross-section, which in the vertical projection on the free hollow section cross-section is congruent with the free hollow section cross-section or lies within the free hollow section cross-section, • wherein the rod blank with compressed blank cross-section is arranged within the free hollow section cross-section in the inner space of the hollow section and • wherein the rod blank arranged in the inner space of the hollow section is supported due to elastic expansion of the compressed blank cross-section on the inside of the hollow section, characterised in that, • the oversized blank cross-section of the rod blank with not yet hardened fibre-reinforced plastic is elastically compressed, outside the hollow section to become a compressed blank cross-section • the rod blank with compressed blank cross-section and not yet hardened fibre-reinforced plastic is arranged within the free hollow section cross-section in the inner space of the hollow section and • that the fibre-reinforced plastic of the rod blank arranged in the inner space of the hollow section and supported due to elastic expansion of the compressed blank cross-section on the inside of the hollow section is hardened thus forming the plastic rod • wherein a rod blank of the plastic rod is made with not yet hardened fibre-reinforced plastic and with an oversized blank cross-section, which extends in the vertical projection on the free hollow section cross-section beyond the free hollow section cross-section and which is elastically compressible and elastically expandable after the elastic compression, • wherein the oversized blank cross-section of the rod blank is elastically compressed outside the hollow section to become a compressed blank cross-section, which in the vertical projection on the free hollow section cross-section is congruent with the free hollow section cross-section or lies within the free hollow section cross-section, • wherein the rod blank with compressed blank cross-section is arranged within the free hollow section cross-section in the inner space of the hollow section and • wherein the rod blank arranged in the inner space of the hollow section is supported due to elastic expansion of the compressed blank cross-section on the inside of the hollow section, characterised in that, • the oversized blank cross-section of the rod blank with not yet hardened fibre-reinforced plastic is elastically compressed, outside the hollow section to become a compressed blank cross-section • the rod blank with compressed blank cross-section and not yet hardened fibre-reinforced plastic is arranged within the free hollow section cross-section in the inner space of the hollow section and • that the fibre-reinforced plastic of the rod blank arranged in the inner space of the hollow section and supported due to elastic expansion of the compressed blank cross-section on the inside of the hollow section is hardened thus forming the plastic rod