To provide a radio wave half mirror for a millimeter wave band which can flatten transmittance characteristics and a method of flattening the transmittance of the radio wave half mirror for a millimeter wave band. A radio wave half mirror 20 includes a metal plate 21 that has an outward shape closing a transmission line 11 and a slit 22 for transmitting electromagnetic waves that is provided in the metal plate 21 along a long side of an opening of the transmission line 11. The thickness L of the metal plate 21 in a direction in which the electromagnetic waves pass through the slit 22 is set on the basis of the transmittance characteristics of the electromagnetic waves.