1. An electronic endoscope system comprising: an imaging device for obtaining image signals through imaging of an interior of a body cavity at constant intervals; an endoscopic image producing device for producing endoscopic images sequentially based on the image signals; a special light projecting device for projecting special illumination light into the body cavity, said special illumination light having a different wavelength range from white light; a tracing target designating device for designating a tracing target in an endoscopic image; a biological information acquiring device for acquiring biological information on the designated tracing target from image signals obtained while said special illumination light is being projected into the body cavity; and a tracing device for tracing the designated tracing target in endoscopic images captured after the tracing target is designated, on the basis of the biological information acquired by said biological information acquiring device wherein said tracing target designating device comprises: a display device for displaying an area designating frame on endoscopic images on a screen; and a lock-on device for allowing a user to designate a portion confined in the area designating frame on said screen as the tracing target.