A capsule endoscope has an imaging lens system that forms an image of a substantially spherical object in a plane shape on the imaging surface. The imaging lens system includes first to fourth lenses and an aperture diaphragm. The surface shapes and arrangement thereof are set in consideration of a front cover of the capsule endoscope and a cover glass of an imaging device. Further, the imaging lens system is configured so that the following conditional expression is satisfied with respect to an arbitrary half angle of view ω: 0.7