An image signal processing device improving quality of three-dimensional image is provided. The device includes a determination section; deinterlace sections and a synchronous control section. The determination section determines whether first and/or second input image signals, having horizontal parallax there between, are interlaced signals derived from video signal or from pull down-converted film signal. The deinterlace sections perform deinterlace on each of the first and second input image signals, through interpolation for a video signal or pull down reverse conversion for a film signal, and generate first and second output image signals as progressive signals, having horizontal parallax there between. The synchronous control section synchronously controls the deinterlace, based on result of the determination section, such that deinterlace process onto the first and second input image signals, synchronized with each other for each of fields, are of same type.