Abstract: The present invention relates to a pneumatically operable fastener driving tool which uses a safety device which prevents the operation of the tool until it is in engagement with the workpiece. More particularly, the safety mechanism includes a safety nose that is normally retained in a retracted position prior to the tool being operated. When the tool is to be operated, the safety nose is extended to be engaged by the workpiece. Subsequent engagement of the safety nose by the workpiece permits the trigger mechanism to effect driving movement of the driver. The mechanism includes a first valve which controls the flow of high-pressure air to the driving piston and a second valve which controls the flow of air to the safety mechanism. The trigger mechanism initially operates the second valve to move the safety nose to the operative position and when the safety nose engages the workpiece it effects operation of the first valve to control the operation of the driving piston.