To constitute an observation section of an image display device to a small size in such a manner that the device is mountable at a portable telephone and portable information terminal.

This portable type image display device has a speaker section and a microphone section and has an image display element 9 and an observation optical system having positive refracting power as a whole forming an exit pupil in order to observe the observation image displayed thereon. The observation optical system comprises at least one prism member 10. The prism member 10 has an incident surface which makes the luminous flux emitted from the image display element 9 incident on the inside of a prism, at least one reflection surface which reflects the luminous flux within the prism and an exit surface which emits the luminous flux to the outside of the prism. At least one reflection surface has a curved shape to impart power to the luminous flux. The curved surface shape is composed of a rotationally asymmetric surface shape for correcting the aberrations generated by eccentricity.