1. A fitness device-based simulator comprising: a processor; and a memory storing units comprising computer executable code configured and executed by the processor, wherein the units comprise: a feature point extraction unit which acquires action-sensing information of a user who is located on a fitness device from one or more camera sensors, and extracts feature points for a body skeletal structure of the user per unit time based on the action-sensing information; a feature point cluster generation unit which generates multiple feature point clusters by, clustering two or more of the feature points extracted by the feature point extraction, unit within a preset period of time, and sets respective cluster symbols for the multiple feature point clusters; an exercise pattern information accumulation unit which generates and stores information about a state transition between the multiple feature point clusters of the user using the cluster symbols of the respective feature point clusters set by the feature point cluster generation unit; and an exercise state prediction unit which predicts a subsequent exercise state of the user by predicting a feature point cluster subsequent to a feature point cluster currently being generated for the user, based on the information about state transition between the multiple feature point clusters for the user, which is previously stored in the exercise pattern information accumulation unit.