To prevent the security troubles by limiting each access level based on the password that is added to every command.

A user previously inputs and stores a prescribed PW(password) into a UI(user interface) device 120a which is connected to an access path 10a. When the user accesses a UP (user program) containing a command whose AL(access level) is limited based on the stored PW, the device 20a sends the UP to a CPU 11b via a communication port 12 together with the PW that is stored for every command of the UP. The CPU 11b collates the received PW with a prescribed PW which is previously stored in a RAM 11c and decides an AL. If the accessed command contained in the UP has an allowance AL, the CPU 11b executes the command. Meanwhile, the command is not executed if it does not have the allowance AL and an access unable response is sent back to the device 20a.