An apparatus for detecting information on an object includes an irradiating unit, a detecting unit, and an information-obtaining unit. The irradiating unit irradiates the object with electromagnetic waves as a rectangular shaped beam. The electromagnetic waves include a frequency component in (or range of) at least part of a frequency region of 30 GHz to 30 THz, and have variations in magnitude at intervals of 10.sup.-11 seconds or more or a temporally constant magnitude. The detecting unit detects the electromagnetic waves transmitted or reflected by the object through interaction therebetween. The information-obtaining unit obtains information on the object from information on the electromagnetic waves detected by the detecting unit.