A network model representation of the laser devices including the associated optical components are described, and a circuit theory for practical laser device design is developed. The theory described in the present paper would be applicable not only to the laser devices, but also to the widespread variety of either passive or active optical devices and systems so long as they are regarded as linear. A number of illustrative examples such as multi-layered dielectric reflectors, Fabry-Perot type lasers, mode-selective cavities, multi-segmented lasers, etc., are treated by means of the above mentioned circuit theory.