The highly reliable distributed system is composed of a communication protocol processing unit which comprises a mailbox for storing a communication message, and executes communication protocol processing between data of an application program and a network controller using the network controller performing network communication of the message in the mailbox; an error detection coding unit; an error detection decoding unit which reconverts data converted from communication data by the error detection coding unit to the original data, and detects that the content of the data is damaged if it is damaged; and a data comparing unit for checking whether or not two kinds of data agree with each other.