1. An endoscope apparatus comprising: an endoscope comprising a solid imaging device; a processor unit which forms an observation image from signals emitted from the solid imaging device, the processor being connected to the endoscope; and one or a plurality of monitors each of which is connected to the processor unit, wherein the processor unit detects the number of connected monitors thereto, when the connected monitor is plural, a whole image of the observation image is controlled to be displayed on at least one of said plurality of monitors in the plurality of connected monitors, and a partial image of the observation image is enlarged and displayed on the other one(s) of said plurality of monitors, and when the connected monitor is single, the monitor is controlled to display the whole image of the observation image wherein if the processor unit detects that the number of connected monitors is plural, the monitors have respective different resolutions, the processor unit obtains a resolution of each of the monitors, and controls the monitors so that a whole image of the observation image can be displayed on a high resolution monitor having the highest resolution and a partial image of the observation image can be displayed on at least one low resolution monitor having lower resolution than the highest resolution, and each one of pixels of at least one of said at least one low resolution monitor can be displayed so as to correspond to each one of pixels corresponding to the partial image of the solid imaging device, by which the partial image can be enlarged and displayed on said at least one of the low resolution monitor, wherein the pixel number of the highest resolution monitor is greater than the pixel number of the solid imaging device, the pixel number of the low resolution monitor is equal to or smaller than the pixel number of the solid imaging device, a pixel size of the low resolution monitor is larger than a pixel size of the highest resolution monitor, and the partial image of the observation image can be displayed on the low resolution monitor without electronic enlargement processing.