PURPOSE: To selectively display the contents of a message issued by an application program corresponding to the skilled level of an operator.

CONSTITUTION: A setting level (skilled level) is stored in a level storage part 22 with a level setting input by the operator. When an application program 23 transmits a message display command along with a message number to a message display control part 24, the message display control part 24 reads the level number corresponding to the message number and the message contents from a message file 25, refers to the level storage part 22, and when the set level coincides with the level number, displays the corresponding message contents on a display 31.