A capsule endoscope includes an image pickup optical system for imaging an object. The image pickup optical system satisfies a condition expressed by −5≦ΔZr/ΔZp≦5. Assuming ωmax is a maximum half angle of view, ΔZr is a difference between a position of a real image surface with respect to light flux of 2ωmax and that with respect to light flux of ωmax, and ΔZp is a difference between a paraxial image forming position of a first virtual object plane surface and that of a second virtual object plane surface. The first virtual object plane surface passes through an intersection point P1 of the object and principal rays of 2ωmax. The second virtual object plane surface passes through an intersection point of the object and principal rays of ωmax. The first and second virtual object planes are vertical to the optical axis.
