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-processor coupled to the first sensor and the second sensor, wherein the pre-processor 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, defines invalid areas of the reference image according to an image range of the each first image, the invalid areas of the reference image are located outside the image range of the each first image, sets a first predetermined gray level value to pixels of the invalid areas of the reference image, and the second image corresponds to the each first image; a depth map generation circuit coupled to the pre-processor, wherein the depth map generation circuit generates a first depth image according to the reference image and the target image; and an error determination circuit coupled to the pre-processor and the depth map generation circuit, wherein the error determination circuit defines invalid areas of the first depth image according to the invalid areas of the reference image, and sets a second predetermined gray level value to pixels of the invalid areas of the first depth image to define invalid areas of a second depth image, wherein the invalid areas of the second depth image are neglected.