PURPOSE: To accurately detect a degree of muscular relaxation, by constituting the title monitor of a fluid sac for imparting electric stimulation in order to contract the muscle relating to the motion of a finger to be deformed in relation to the motion of the finger and a pressure sensor for detecting the pressure in the fluid sac.

CONSTITUTION: When a calibration switch 34 is operated in such a state that a muscle relaxant is not administered to a patient, a controller 26 imparts stimulation according to the stimulation system selected by a setting device, for example, either one of single stimulation, tetanic stimulation and train-of-four stimuli systems. When the ulnar nerve is stimulated by the selected stimulation system, the thumb reflectively moves to deform an air bag 18. The controller 26 calculates the pressure change in the air bag 18 on the basis of the signal outputted from a pressure sensor 20 at this time and stores the intensity of the motion (magnitude of the reaction) of the thumb in an unrelaxed state and calculates the evaluation value corresponding to the stimulation system to set the same, for example, to a degree of muscular relaxation of 100%. After calibration is completed, the degree of the muscular relaxation of the patient to which the muscle relaxant is administered is quantitatively monitored.