Suitability of an operational state of a steam-using equipment is accurately estimated to allow for early detection of a sign of abnormality in the steam-using equipment. A steam-using facility monitoring system composed of a steam-using equipment includes a control state detector that detects a state of a steam controller provided in a steam pipe accompanying the steam-using equipment, and an operational state estimating means having a signal input unit that inputs a detection signal from the control state detector and an operational state estimation unit that estimates the operational state of the steam-using equipment based on one inputted detection signal or a preset particular combination of detection signals.