A fastening tool comprising: a housing (2, 302) defining a first air chamber (2a, 302a) for accumulating compressed air; a trigger (12, 312) provided at the housing (2, 302); a cylinder (3, 303) accommodated in the housing (2, 302); a piston (4, 304) accommodated in the cylinder (3, 303) and dividing the cylinder (3, 303) into a cylinder upper chamber (35, 337) and a cylinder lower chamber (36, 338); and a main valve (5, 305) configured to allow a communication between the cylinder upper chamber (35, 337) and the first air chamber (2a, 302a) in conjunction with an operation of the trigger (12, 312), wherein the housing (2, 302) is formed with a second air chamber (34, 334) and a third air chamber (33, 333); the second air chamber (34, 334) is communicated with the cylinder upper chamber (35, 337) in conjunction with a movement of the piston (4, 304), characterized in that: the third air chamber (33, 333) accumulates or discharges the compressed air in the first air chamber (2a, 302a) in conjunction with the operation of the trigger (12, 312); and the cylinder (3, 303) is movable between a first position where the cylinder upper chamber (35, 337) is in communication with the first air chamber (2a, 202a), and a second position where the cylinder upper chamber (35, 337) is blocked from the first air chamber (2a, 302a) based on a differential pressure between the second air chamber (34, 334) and the third air chamber (33, 333). a housing (2, 302) defining a first air chamber (2a, 302a) for accumulating compressed air; a trigger (12, 312) provided at the housing (2, 302); a cylinder (3, 303) accommodated in the housing (2, 302); a piston (4, 304) accommodated in the cylinder (3, 303) and dividing the cylinder (3, 303) into a cylinder upper chamber (35, 337) and a cylinder lower chamber (36, 338); and a main valve (5, 305) configured to allow a communication between the cylinder upper chamber (35, 337) and the first air chamber (2a, 302a) in conjunction with an operation of the trigger (12, 312), wherein the housing (2, 302) is formed with a second air chamber (34, 334) and a third air chamber (33, 333); the second air chamber (34, 334) is communicated with the cylinder upper chamber (35, 337) in conjunction with a movement of the piston (4, 304), characterized in that: the third air chamber (33, 333) accumulates or discharges the compressed air in the first air chamber (2a, 302a) in conjunction with the operation of the trigger (12, 312); and the cylinder (3, 303) is movable between a first position where the cylinder upper chamber (35, 337) is in communication with the first air chamber (2a, 202a), and a second position where the cylinder upper chamber (35, 337) is blocked from the first air chamber (2a, 302a) based on a differential pressure between the second air chamber (34, 334) and the third air chamber (33, 333). the third air chamber (33, 333) accumulates or discharges the compressed air in the first air chamber (2a, 302a) in conjunction with the operation of the trigger (12, 312); and the cylinder (3, 303) is movable between a first position where the cylinder upper chamber (35, 337) is in communication with the first air chamber (2a, 202a), and a second position where the cylinder upper chamber (35, 337) is blocked from the first air chamber (2a, 302a) based on a differential pressure between the second air chamber (34, 334) and the third air chamber (33, 333).