PURPOSE: To completely save labor by automatically changing over the set and reset conditions of a stopper piece in a material positioning device which positions the material which is carried in an open tip machining device.

CONSTITUTION: In the condition before a material 51 is carried in, a stopper piece 35 is moved into the set condition where a material 51 advances into a carry-in passage 13 due to gravity of a positioning member 32. For this reason, when the material 51 is carried into the carry-in passage 13, end face of the material contacts the stopper piece 35 of the positioning member 32 and is positioned. When a cutting head is moved to cut the positioned material 51, a guide member 21 of the cutting head contacts a contact section 46 of the positioning member 32 and presses. The positioning member 32 is automatically moved into the reset condition where the stopper piece 35 retracts from the carry-in passage 13. When the cutting head is restored at wait position after cutting is done, it is automatically moved into the set condition where the stopper piece 35 advances into the carry-in passage 13 of the material 51.