An optical fiber-coupled type laser has been optimized by taking account of field disturbance on a coupling aperture mirror, and this has been examined by using conventional CO2 laser. Solving the eigen equations for a quasi-hemispherical resonator with a coupling aperture to find the field distribution on the plane mirror with an aperture, its effective transmittance is analytically evaluated as a function of normalized aperture ξ by the mirror size and is well approximated by the power series expansion up to the fourth order for various mirror Fresnel numbers N and the g parameter of resonator. Using this obtained and typical laser parameters for the experimental set to be used, the coupling aperture is optimized to yield the maximum power. As a result, the optimum aperture becomes a minimum at a specific g2 parameter of the concave mirror, i.e. g20.73, implying that the optimum configuration corresponds to be slightly off from a pure hemispherical setting. These theoretical estimates of the output power vs aperture have been compared with the experimental results obtained for a CO2 laser, being in qualitative agreements with each other in overall characteristics. By this optimization the maximum output power 6.0 W was extracted from the aperture of 2.4 mm.