The film dose distribution method is used for profile measurements of non-physical wedges, because three-dimensional automatic control water phantoms cannot be used. Recently, many hospitals have adopted computed radiography(CR) systems in preference to automatic developing processors and films. This may allow use of automatic processors to be discontinued. In this study, a beam was irradiated to an Imaging Plate(IP), and then IP was exposed to a fixed amount of light with fading, and we then measured the off-center ratio(OCR) absorbed dose distribution in external radiation therapy with non-physical wedge. This was compared with the OCR measured with an ionization chamber dosimeter. It was consequently possible for IP to approximate the value measured by the ionization chamber dosimeter by using a metal filter. This method offers a simple quality assurance technique for absorbed dose distribution in external radiation therapy with non-physical wedges in consideration of the character of the IP.