1. An in-vivo image acquiring apparatus to be taken into a body of a subject, comprising: an imaging unit that acquires in-vivo image information; an illuminating unit that illuminates imaging regions of which images are taken by the imaging unit; a radio transmitting unit that transmits the image information acquired by the imaging unit to an external apparatus by radio; an operation control unit that controls operations of the imaging unit and the illuminating unit so as to control a black image acquiring operation in which the imaging unit conducts an imaging operation with the illuminating unit not conducting an illuminating operation; an average value calculating unit that calculates an average value of pixel values of a predetermined determining area among the image information acquired by the black image acquiring operation; and a black image determining unit that determines whether the image information acquired by the black image acquiring operation indicates a black image by comparing the average value calculated by the average value calculating unit with a predetermined threshold value, wherein the radio transmitting unit transmits to the external apparatus by radio the image information which is determined by the black image determining unit as the black image, as black image information.