By inserting an As2S3 optical fiber directly into a coupling hole of one of the optical resonator mirrors, a pigtail-type CO laser has been created. Its fundamental operation characteristics are described, together with theoretical treatments. First, by finding the spatial intensity distribution on the coupling-hole mirror, its effective transmittance t is evaluated in comparison with the measured spatial beam profile. Then, the output power is measured with various sizes of coupling-hole mirrors, obtaining the maximum output power at a coupling-hole diameter of ?0.6 mm. The dependence of the output power on the coupling-hole diameter (or the effective transmittance t) is theoretically explained by using the gain parameter G, and the internal loss ã, is obtained in good agreement with the experiments. It follows that the optimum coupling- hole diameter as evaluated is quite suitable for such a laser. Possible applications of this scheme to computer-aided robotics and automatic machinery are also described.