An apparatus configured to acquire information on an object to be measured by an electromagnetic wave pulse, the apparatus includes: a generating unit configured to generate the electromagnetic wave pulse with which the object to be measured is irradiated; a detecting unit configured to detect the electromagnetic wave pulse from the object to be measured; a casing including at least a part of a propagation path of the electromagnetic wave pulse leading from the generating unit to the detecting unit; and a measuring window unit configured to change a propagation distance of the electromagnetic wave pulse by moving a measuring window disposed in a part of the casing. The object to be measured is disposed on an opposite side of the propagation path of the electromagnetic wave pulse inside the casing by interposing the measuring window.