We investigated the nature of impression manipulation of gender differences on 3D faces. First, we investigated the differences in gender impressions when observed from different viewpoints for a 3D face that had performed masculinity / femininity conversion operations at various intensities. As a result, it was found that the difference in impression due to gender was emphasized when the face was viewed from an angle. In addition, studies are being conducted on gender difference determination using a facial image sex determination system based on machine learning for a three-dimensional face obtained from masculinity / femininity impression conversion.