1. An object detection device, comprising: an obtaining unit configured to successively obtain frame images; an image storage unit configured to temporarily store the frame images, which are obtained by the obtaining unit at different times; a first similarity calculating unit configured to calculate a first similarity between (i) an image of a specific region in a first frame image stored in the image storage unit and (ii) a reference image, wherein the reference image is a criterion for determining a specific object image; a first determination unit configured to determine whether the first similarity calculated by the first similarity calculating unit is equal to or more than a first threshold value; a second similarity calculating unit configured to, when the first determination unit determines that the first similarity is equal to or more than the first threshold value, calculate a second similarity between (i) an image of a specific region in a second frame image stored in the image storage unit and (ii) the reference image, wherein the specific region in the second frame image corresponds to the specific region in the first frame image, and the second frame image is a frame image which is obtained a predetermined number of frames prior to the first frame image; a second determination unit configured to determine whether the second similarity calculated by the second similarity calculating unit is equal to or more than a second threshold value larger than the first threshold value; and a detection unit configured to detect the specific region of the first frame image as a region of a particular object image when the second determination unit determines that the second similarity is equal to or more than the second threshold value.