To provide a concrete reinforcing work method which achieves reinforcement without covering the entire area of a beam and a lower surface of a floor slab of a viaduct.

In the work method, a cover concrete portion 5 on the lower surface 1c of the floor slab 1 has grooves 7 formed therein, each having a width slightly larger than a diameter D1 of a fiber reinforced plastic rod 8. Then the fiber reinforced plastic rod 8 is embedded in the groove 7 via an adhesive material 10. The entire area of the lower surface 1c of the floor slab 1 is not covered, and therefore water infiltrated into the floor slab 1 is not retained in the floor slab 1, which prevents degradation of the long-term durability of the concrete due to water retention. Further, since cracks etc. on the lower surface 1c of the floor slab 1 can be visually observed, degradation of the concrete and development of damage can be easily visually checked.