PURPOSE: To prepare a program through the use of a DDC instruction which is freely prepared and registered with POL, etc., by a user at the time of programming the instrumentation control, etc., of a controller.

CONSTITUTION: At the time of selecting a kind including pertient POL from the kinds of POL displayed at a tool bar 16 in a DDC instruction editing picture, POL instructions included in the kind are displayed at a sub-tool bar 17. Then, a necessary instruction is selected from them to prepare an optional DDC instruction. Programming for instrumentation control is made possible by using the DDC instruction registered by the picture. Thereby, the DDC instruction prepared by the user can be used so that programming is easy and using memories are reduced.