PURPOSE: To attain a program test with plural tasks in a single unit or in combination by preparing a task number table where the task numbers to be tested are registered among plural tasks.

CONSTITUTION: A task 4, for example, is registered to a task number table 9 for start of its test before the program tests are started with tasks 4 and 5. Then the task 4 gives an access to a data file 3 through a common subroutine 8. The routine 8 refers to the table 9 and delivers the result of this reference to a simulation program 6 and then to a printer 7. Then the task 5 gives an access to the file 3 through the routine 8 and this routine 8 refers to the table 9. However the task 8 delivers no result of said reference to the program 6 to complete its operation since the task 5 is not registered to the table 9.