A lamp device includes a lamp seat defining a first through hole, a lamp main body defining a second through hole, and a holding mechanism. The holding mechanism includes a bolt, a first nut, and a second nut. The bolt includes a head and a spindle connecting to the head. The spindle defines first screw threads at the opposite end from the head and second screw threads adjacent to the first screw threads. The threads per inch of the first screw threads are smaller than that of the second screw threads. The spindle passes though the first through hole and the second through hole to connect with the lamp main body and the lamp seat. The first nut and the second nut are respectively fit over the first screw threads and the second screw threads.