At least an electroencephalogram (EEG) detected by a sensor is input from a signal input section as a vital sign signal. The vital sign signal is stored in a storage as vital sign data. A display includes a first display area, a second display area and a third display area. A controller reads out the vital sign data from the storage to display the vital sign signal on the display. The controller includes a first display processor, which displays the EEG in a real time manner, on the first display area, a second display processor, which displays the EEG at a time point in past, on the second display area, and a third display processor, which displays a time-varying trend of a parameter of the EEG, on the third display area. A designator designates a time point in past in the trend displayed in the third display area, so that the EEG at the designated time point is displayed on the second display area.