To uniformly control cutting wire tensions between groove rollers at the time of reciprocatingly feeding by adding a tension regulating means for regulating the tensions, and substantially equalizing a tension of input side of the wire to the roller to that of output side.

In a tension control mechanism 19a of an input side, a cylinder body 21 of an air cylinder 20 is mounted at a nearest position to one end of a main weight 18a fixedly held at an arm 12a for supporting a dancer roller 10a such as, for example, a view point 11a. Here, a rod 22 approachably to or retractably from the arm 12a in a lengthwise direction is provided, and a subweight 23a having a lighter weight than that of the weight 18a is connected to the end. Thus, moment of the roller 10a in the case of oscillating the roller 10a is changed in response to a feeding position of the subweight 23a, and a tension of the wire is regulated via the roller 10a. Incidentally, similar constitution is formed even in a tension control mechanism 19b of an outputs side.