The present invention provides an imaging unit, a portable terminal device, and a portable terminal system capable of performing a satisfactory key synthesizing process. An imaging unit mainly includes an imaging section, a conversion section, and a key signal generating section. The conversion section converts the format of the imaged image data output from the imaging section from YUV format to RGB format. The key signal generating section generates a key signal based on each pixel data configuring the imaged image data and the reference data for the imaged image data input from the imaging section. The key signal generating section also outputs foreground image data having the generated key signal and the corresponding pixel data of RGB format as minimum configuring unit. An image synthesizing section of a main unit generates synthesized image data by overlapping the foreground image data from the imaging unit and the background image data stored in a RAM based on the key signal contained in the foreground image data.