PURPOSE: To prevent the breakdown of a system by providing a stand-by unit to which an operating unit can be switched if a trouble detecting circuit, which is provided for n-number of operating units which process n-byte operations simultaneously in parallel, detects a trouble.

CONSTITUTION: N-number of read registers 11W1n read out data in byte units from a memory, which is not shown in figure and store data. Data in registers 11W1n are processed simultaneously in parallel by n-number of current use operating units ALUs 21W2n, and processing results are written in write registers 31W3n. Each of operating units 21W2n has a parity check circuit PC, and outputs of circuits PC are inputted to a selecting circuit 4 consisting of n-number of FF circuits 41W4n. The FF circuit 4i corresponding to the ALU2i where a trouble occurs issues the output to open gates of input and output switching circuits 5 and 6 of a stand-by ALU20. Input gates of the other FF circuits are closed by the inverted output of the turned-on FF circuit 4i. Thus, the breakdown of the system is prevented.