There is provided a surface light source device having a configuration where light sources are arranged on both of a pair of side surfaces of a light guide plate and allowed to independently form nonuniform light emission intensity distributions in which light emission intensity is lower and higher at places closer to and farther from each of the light sources. A synchronization signal is applied to the bus and a number of data volume counters corresponding to the number of transmitters reduced by one is set to a predefined initial value. The drum is sealed, and superatmospheric pressure is applied therein to drive the treatment chemicals into the waste particles.