An unexpected demand detection system for detecting an unexpected demand for IT resources includes: a demand model recording part that records a demand model representing a time-based transition in a predetermined time period of a required amount of the IT resources; an unexpected demand model recording part that records an unexpected demand model representing a time-based transition different from the time-based transition of the amount of the IT resources represented by the demand model; and a judging part that performs a comparison among observation data indicating an actual demand for the IT resources observed in the predetermined time period, the amount of the IT resources represented by the demand model, and an amount of the IT resources represented by the unexpected demand model thereby to judge whether an unexpected demand represented by the unexpected demand model occurs.