To provide a yarn float sewing device for performing yarn float sewing without limiting yarn floating amounts.

A button sewing machine 1 vibrating a needle relatively to the sewing needle 3 is provided with at least one of a needle vibrating means for vibrating a sewing needle 3 and a button feeding means capable of moving a button in the horizontal direction. This yarn float sewing device is disposed on the above button sewing machine, and provided with a loop spreading pin 71 drawing substantially horizontally above a button above the button, a pulse motor 72 for reciprocating the loop spreading pin 71 in the needle vibrating direction to cross above the needle drop position in a button hole, and a control part 73 for controlling the drive of the pulse motor 72 to reciprocate the loop spreading pin 71 in the needle vibrating direction at every needle vibration.