A prestressing steel material for use with concrete that is prestressed by posttensioning is disclosed. Said steel material is unbonded from the concrete. The prestressing steel material is composed of a steel member sheathed with a foamed synthetic resin tube. The wall thickness of the synthetic resin tube is at least 300 microns, more preferably, more than 500 microns. In the case that the steel member is a strand composed of a plurality of twisted steel wires, the spiral grooves of the strand are first filled with a resin and the strand together with the resin sheathed with the foamed synthetic resin tube.