A nail-driving device includes a housing, a trigger unit, a safety unit, a connecting unit, and a magazine. The safety unit includes first and second safety members disposed movably in the housing. The connecting unit includes a retaining portion disposed movably between the first and second safety members and in a retaining position whereat a force is applicable to the first safety member to move the second safety member to pivot a trigger arm of the trigger unit so as to allow for a nail-driving operation when the trigger unit is actuated. The magazine includes a pusher for pushing and moving nails in the magazine. The pusher is movable to a retention releasing position whereat the retaining portion of the safety unit is removed from the retaining position to thereby allow for relative movement of the first and second safety members so that a nail-driving operation is prevented.