To simplify and compact constitution and to improve reliability, productivity and operability.

A level difference pin 5 is provided with a large diameter part and a small diameter part, the large diameter part of the level difference pin 5 is inserted to the forth hole part 9c of a second vertical feed arm 9 at the time of turning a feed dog 6 to a floating state and the small diameter part of the level difference pin 5 is inserted to the forth hole part 9c at the time of turning it to a sunk state. Thus, a clearance is formed between the forth hole part 9c and the small diameter part of the level difference pin 5 in the sunk state, the second vertical feed arm 9 is turned for the clearance and the feed dog 6 is sunk through a supporting pin 8 connected to the second vertical feed arm 9.