A main optical system for an endoscope especially related to the main optical system for an endoscope capable of confocal imaging of visible light and near infrared light belongs to the technical field of minimally invasive medical treatments. The optical system comprises: an object lens group (1) having positive defocus and a positive curvature of field an image transmission group (2) having negative defocus and a negative curvature of field a field lens group (3) added between the object lens group (1) and the image transmission group (2), and located near the focus of the object lens group (1). After the combination of the field lens group (3) and the object lens group (1), positive defocus and a positive curvature of field matched with the negative defocus and the negative curvature of field of the image transmission group (2) is obtained. The defect of difficult to correct an aberration in a wide spectral range by the object lens group (1) or the image transmission group (2) individually can be effectively solved. Compared with the traditional HOPKINS transmission system, the image transmission group (2) has the advantage of easy to manufacture, easy to control the precision, and reducing the cost. The optical system is suitable for minimally invasive surgery using IR fluorescence, so as to solve the problem of focusing repeatedly when the traditional endoscope is used in the surgery.Linvention porte sur un système optique principal pour un endoscope, spécialement associé au système optique principal dun endoscope apte à une imagerie confocale de la lumière visible et de la lumière du proche infrarouge, lequel système appartient au domaine technique des traitements médicaux à effraction minimale. Le système optique comprend : un groupe de lentilles dobjet (1) ayant une défocalisation positive et une courbure de champ positive un groupe de transmission dimage (2) ayant une défocalisation négative et une courbure de champ négative un groupe de lentilles de champ
