A lamp device includes: a housing formed with heat-dissipating holes; a conductive connecting head mounted on a first side of the housing; a heat-conducting member mounted on a second side of the housing opposite to the first side, and having opposite first and second side surfaces; a lighting unit thermally contacting and mounted on the first side surface of the heat-conducting member, and covered by a transparent body; and a heat-dissipating layer made of an infrared radiating material. The heat-dissipating layer is disposed on and is in thermal contact with the second side surface of the heat-conducting member. Heat generated by the lighting unit is transmitted by the first heat-conducting member to the heat-dissipating layer, and is dissipated by the heat-dissipating layer through the heat-dissipating holes in the housing.