A lamp device includes a heat dissipating seat, a light emitting unit, a mounting seat and a light guide plate. The heat dissipating seat defines an accommodation space with an opening. The light emitting unit is fixed to the heat dissipating seat within the accommodation space. The mounting seat is disposed in the accommodation space through the opening, and covers the light emitting unit. A receiving space is defined by at least a portion of the mounting seat and extends in a direction away from the light emitting unit. The light guide plate has at least a portion inserted into and fixed within the receiving space.