An image processing system, an imager, an area detection method, and a computer program easily and highly accurately detecting an area where a target object exists in an image acquired by imaging the target object are provided. The image processing system includes a projector for projecting a projection image having a stripe pattern or a grid pattern toward a target object, and an imager for imaging a target object on which the projection image is projected, and the imager includes an input image obtaining module for obtaining an input image acquired by imaging a target object on which the projection image is projected, a height information calculator for calculating height information of each pixel in the input image by using the input image, and a target object area detector for detecting a target object area where the target object exists in the input image, based on the height information of each pixel in the input image.