To heighten the feed in the case of a low rotational frequency of a driving shaft and inhibit the feed from being further heightened in proportion in the case of a high rotational frequency of the driving shaft.

A transmission gear 60 is a conical ring transmission gear, and the conical transmission gear has a friction conical wheel 62 coupled to the driving shaft 32 and a friction conical wheel 64 coupled to a driven shaft 36. Both friction conical wheels 62, 64 are mutually rolled through a conical ring 66 disposed therebetween, whereby the conical ring 66 is operated as follows in relation to the feed pressure produced by a feed pump 30 to change the transmission ratio of the transmission gear 60. That is, the conical ring is moved so that in the case of low feed pressure, a large transmission ratio of the transmission gear is controlled by the conical ring 66, and with increase in feed pressure, a smaller transmission ratio of the transmission gear 60 is controlled.