With the rapid penetration of the Internet and the development of technology of connecting all of the Internet-based things, the fourth industrial revolution has emerged.  One of various technological fields leading the fourth industrial revolution is a technology related to smart factories. 
The term “smart factory” refers to the general technology that enables machines and facilities in a factory to be connected and easily managed.  One of those technologies for smart factory is a technology of managing machines.  That is, it is a technology of managing machines by monitoring whether machines are normally operated and predicting the occurrence of error. 
Conventionally, in order to predict a malfunction of a machine, a threshold defined as an absolute value irrelevant to time is set and operation data of the machine are frequently collected, and if any operation data deviate from the threshold, it is determined that a specific error occurs.  Conventionally, the threshold is a predetermined allowable value for the machine for production.  In other words, the threshold is a combination of operational upper and lower limits to avoid producing defective products.  Further, conventionally, the existing threshold is a fixed value directly set by an operator in the factory (i.e., an absolute value fixed in each range set by the operator regardless of time).  Therefore, if the operator wants to lower a defect rate by lowering the quality of products, he/she just needs to extend the range of upper and lower limits of the threshold and in the inverse case, he/she just needs to narrow the range of upper and lower limits of the threshold.  In addition, the threshold is set for a time section in which products are produced during a period corresponding to a single operational cycle of the machine, but a separate threshold is not set for a time section irrelevant to the production of products.  Therefore, even though it is possible to determine whether a product has a defect, it is difficult to accurately determine whether the machine has encountered an error or not.