A x-ray preionized XeCl laser with a large active volume has been developed. Use of a transformer between the rail-gap switch and the laser electrode could realize high charging voltage of pulse forming line and a low output impedance. This laser system produced output energy of more than 20 J in a laser-pulse of-90 nsec (FWHM). The maximum output energy of 33 J was extracted from an active volume of 7.7 1 with an electric efficiency of 1.3%.