The invention relates to a front underrun protection device for a heavy vehicle comprising a beam configured to extend from side to side of the vehicle at a lower front position thereof and to absorb collision energy. The beam is at least partly made of a plastic or polymeric material. The beam comprises a housing beam member having at least one channel extending along the housing beam member, and at least one reinforcement beam member adapted to fit into the channel of the housing beam member. The reinforcement beam member is arranged in the channel and arranged to extend along the housing beam member. The reinforcement beam member is provided with a plurality of wall elements comprising transversal wall elements distributed along and extending across the reinforcement beam member and longitudinal wall elements extending in a longitudinal direction of the reinforcement beam member between the transversal wall elements.

A bumper beam body 13 and upper and lower parts of a flange portion 22 each contain a continuous fiber reinforced polymer layer 20, and a first flange tip portion 22a made of thermoplastic resin that has an L-shaped cross section extending outward in a vertical direction and then curving outward in a vehicle rear-front direction is connected to edges of the flange portion 22. Thereby, even when continuous fibers and matrix resin of the continuous fiber reinforced polymer layer 20 stick out from the edges of the flange portion 22 as flash, this flash can be covered with the first flange tip portion 22a.