1. An image device for determining an invalid depth information of a depth image, the image device comprising: a first sensor capturing first images; a second sensor capturing second images; a pre-processing module coupled to the first sensor and the second sensor, wherein the pre-processing module generates a reference image according to each first image of the first images and a target image corresponding to the reference image according to a second image of the second images, and sets gray level values of pixels of the reference image outside an image range of the each first image to be a first predetermined value to determine invalid areas of the reference image, wherein the second image corresponds to the each first image; a depth map engine coupled to the pre-processing module, wherein the depth map engine generates a first depth image according to the reference image and the target image; and an error determination unit coupled to the pre-processing module and the depth map engine, wherein the error determination unit utilizes the invalid areas of the reference image to determine invalid areas of the first depth image, and sets gray level values of pixels of the invalid areas of the first depth image to be a second predetermined value to generate a second depth image, wherein pixels of the second depth image with the second predetermined value have the invalid depth information.