A parallel data processing system processes data by synchronously operating a plurality of data processing units (processor elements). It aims at reducing the overhead caused by the data transmission in a system, performing a matrix operation and a neurocomputer operation by making the best of its parallel processing method, and at using excess units for another operation when the number of units required for an operation is smaller than the number of the existing units. The parallel data processing system comprises a plurality of data processing units; a plurality of trays which store and transmit data, each connected to a data processing unit; a tray connection switching unit for changing the connection state of the data transmission path between trays, dividing data processing units into a plurality of groups, and performing an independent operation on each group; and a clock generator for synchronously operating a data transmission between trays and a data process in a data processing unit. Thus, the data are transmitted while the data are processed, so the data transmission time can be actually counted as zero.