Dependences of laser output energy and excitation power on gap length (14 to 21mm) and ambient pressure (2 to 3atm) are discussed experimentally on an UV-preionized discharge-pumped KrF excimer laser with a charge transfer system for a gas mixture of Kr/F2/He/Ne=2.0/0.25/14.45/83.3%. The specific laser output energy increases with the excitation power, of which relation is expressed in a straight line. It is clarified that the same linear relationship is found in spite of the change of the gap length and ambient pressure. A similarity relationship between the excitation power per unit pressure and the product of gap length and ambient pressure is also clarified. In the case of pressure varied from 2 to 3atm, the increase of excitation power brings larger laser output energy. As the gap length is increased, the increase of the discharge volume also contributes to the laser output energy in addition to the effect of the excitation power. By theoretical discussion with a computer simulation code in the wider range of the gap length and pressure, the existence of an optimum gap length and pressure for obtaining the largest excitation power is clarified. At this case, energy transfer efficiency from a primary capacitor to a secondary capacitor is shown to be about 60 percent.