An automatic measuring system was developed to perform simultaneous measurements of particle size and charge distributions in a clean room and other aerosol sources. The system consists of an electrostatic condenser (EC) and an optical particle counter (OPC), which are controlled by a personal computer (PC). The PC automatically varies the voltage on the EC and converts the corresponding OPC counts to charge distributions. The reliabilities of the data inversion method and the automatic measuring system were confirmed by theoretical and experimental examinations, respectively. In the theoretical examination, inverted charge distributions were tested and compared with various assumed input distributions. In the experiment where monodisperse aerosols neutralized by a 85Kr neutralizer were used, the measurement results agreed well with the theoretical Boltzmann charge distribution. The application of this technique is demonstrated by the measurement of charge and size distributions of aerosols in a clean room.