To realize the output adjustment for a reliable fastener driver to be directly operated by the manual operation.

This fastener driver is provided with a guide cylinder 3 arranged in a housing 2, a propulsive piston 4 arranged in the guide cylinder 3 freely to be displaced in the axial direction, a guide member 5 continued to the guide cylinder 3 on the driving direction side, a presser sleeve 8 for concentrically surrounding at least a part of the guide member 5 from the outside thereof, and a hollow cylindrical elastic buffer element 7 for surrounding a shaft 6 of the propulsive piston 4 in the guide cylinder 3. The presser sleeve 8 is formed of an inside sleeve 9 and an outside sleeve 10 for concentrically surrounding a part of the inside sleeve 9. These sleeves 9 and 10 are connected to be rotated in relation to each other, and displaced for adjustment to two different axial directional terminal positions 11a and 11b.