To realize a multi-step stretching which does not generate a stretching unevenness, by a method wherein in a heating layer between a feeding pinch roll and a take-off pinch roll, a roll which rotates with a fixed rotation sped ratio, is installed, and a polyolefin sheet is brought into contact with the roll, and is stretched while receiving a frictional force from the roll.

A stretching furnace 10 is arranged between a feeding pinch roll 8 and a take-off pinch roll 9, and the inside of the furnace is kept and a fixed temperature. In the furnace, a plurality of, preferably, four to ten rolls 1-7 are arranged. The rolls 1-7 are interlock-connected so that they may rotate with a fixed speed ratio, and the sped ratio of each roll with the next roll is set at a range of 1-2. The speed ratio between the feeding pinch roll 8 and the take-off pinch roll 9, i.e., a total stretching multiplying factor, is preferred to become a value for which the speed ratios of respective rolls are multiplied by each other, and normally, a stretch of 2-40 times is performed. The material of the surface of the rolls 1-7 is made a material of which the frictional factor with a sheet is high, and it is preferable to apply a rubber lining on a metal roll.