PURPOSE: To restart interrupted inference from knowledge data in the execution order held just after the interruption by sequentially saving inference information data obtained as the result of the inference to an external storage device.

CONSTITUTION: Inference information data obtained by inference using individual knowledge data are saved and stored in the external storage device 12 by an inference information data saving means 15. In the case of starting the inference from the knowledge data on the way of the execution order, an inference information data existence checking means 16 checks whether inference information data obtained by the inference using the knowledge data in the execution order preceding the knowledge data to start the inference are saved in the device 12 or not, and when the data concerned is saved, an inference information data loading means 14 loads the inference information data to a main storage device 11 and an inference device 13 advances the inference by using the loaded data. Since the inference information data obtained up to now by the inference is saved to the external storage device 12, the inference can be restarted from the knowledge data on the way of the execution order.