A patient position monitoring apparatus capable of monitoring the position of a patient is provided. The patient position monitoring apparatus includes a position guider emitting a source and a plurality of guide markers sensing the source emitted from the position guider and transmitting sensed information to the position guider. The position guider determines whether positions of the guide markers are changed through the sensed information. When the position of the patient is changed, and thus the source emitted from the position guider is out of the range of the guide markers, a warning signal is output. Accordingly, a user can monitor a variation in the position of the patient even if the user does not continuously observe the patient.