PURPOSE: To strengthen the part near to the part where a fork of a deck board is inserted, in particular a bending rigidity thereof, by providing a reinforcing beam having a heat-melted part on the same plane as the bottom of the beam at least at the one side of the beam, near the beam on the extended line of reinforced ribs.

CONSTITUTION: Grate type reinforcing ribs 10, provided in the fork-insertion part 4a of the deck board, are thicken and the other reinforcing ribs are thinned both in the upper and lower plates. Beams 11 having triangular sides 11a parallel to the upper and lower plates at the top thereof for instance are formed on the extended line of ribs 10a perpendicular to the longitudinal direction of the fork-insertion part 4a among the reinforced ribs 10 provided in the fork- insertion part 4a in particular, in the near part of beams 8, 9. Similarly, other reinforcing beams 12, are provided also, having sides 12a of similar shapes to the above shape and these beams are made small not to hinder the insertion of the forks. These beams 11, 12 are connected with heat at the part of the sides 11a, 12a on combining the upper plate with the lower plate.