PURPOSE: To form a reliable path between a terminal equipment and a data processing unit by using a reliable shell process for a program to call a child init program so as to make exec system call.

CONSTITUTION: A child init process uses a reliable shell process TSH9 as a called program to make exec system call. The reliable shell process TSH is allowed for access to drivers DD(A) and KD(A) based on access permission tables AT(A) and AT(A'). Since other process than the reliable shell process TSH9 cannot be connected to a character buffer of the terminal equipment A, the reliable path from the terminal equipment A to the reliable shell process TSH9 is established. Thus, safe communication is attained between the user terminal equipment and the reliable shell process in the inside of the data processing unit without adverse effect on the data processing on progress from other terminal equipment connecting to the same data processing unit.