A main spindle monitoring device determines an abnormality of a main spindle in a machine tool. The main spindle monitoring device includes a main spindle operation monitoring unit and a feed axis operation monitoring unit. The main spindle operation monitoring unit monitors a change of an operation condition of the main spindle based on a main spindle load. The feed axis operation monitoring unit monitors a change of an operation condition of a feed axis based on a feed axis load. The abnormality of the main spindle is determined when the main spindle operation monitoring unit detects the change of the operation condition of the main spindle and the feed axis operation monitoring unit does not detect the change of the operation condition of the feed axis in a state where a constant rotation speed is commanded to the main spindle while the feed axis is driving.