A front side member having a body formed from a tubular body which has a closed cross section and which is constituted by a single member in the axial direction, said body having, from one end towards the other end in the axial direction, a front portion which extends in the fore and aft direction of a vehicle body, a sloping portion which is continuous with the front portion and which slopes downwards along a dash panel, and a rear portion which is continuous with the sloping portion and which extends along the bottom surface of a floor panel which is joined to the dash panel, characterized in that
- a portion of the front portion is an unquenched portion which has not undergone quenching treatment and the remainder other than that portion is a high frequency quenched portion which has undergone high frequency quenching,
- all of the sloping portion is a high frequency quenched portion which has undergone high frequency quenching, and
- a portion of the rear portion is an unquenched portion which has not undergone quenching, and the remainder other than that portion is a high frequency quenched portion which has undergone high frequency quenching.