It was proposed that there are two kinds of apparent size distortion to be distinguished in the Ponzo illusion. The apparent size of the single line in the converging lines (CL) became larger as it was located nearer to the apex. However, this distortion could not predict the obtained illusion sufficiently. Hence there should be assumed another kind of size distortion in the perception of the size relation as a result of comparison of two paralle lines (PL). In this comparison process, breaking of the continuity of CL and elimination of the oblique components from CL had a slight decreasing effect on the illusion. Moreover, it was found in the various contour figures that the greater the separation of PL was, the larger the illusion was.