A decentering optical system which takes a substantially parallel beam as input light, includes a prism having a medium whose refractive index is greater than 1, and four optical surfaces disposed at boundary surfaces of the medium so as to be mutually decentering or to slant. The four optical surfaces form a light-incident surface through which the input light enters, at least two internal-reflective surfaces which reflect internally the input light entering inside, a splitting surface which splits an optical path of the input light reflected by any one of these internal-reflective surfaces, into a transmission optical path and a reflection optical path; and a transmitting surface which emits the input light traveling along the reflection optical path towards an exterior. Real images are formed along each of the transmission optical path and the reflection optical path.