A radio-wave transmitting cover is configured to be arranged in the path of the radio wave of a radio-wave radar device. The thickness of the cover is set to a reference thickness at a reference position where the incident angle of the radio wave from the radio-wave radar device is 0°. The reference thickness is set to a value obtained by multiplying half of the wavelength of the radio wave by an integer. In the area around the reference position, the thickness of the cover is gradually changed such that the greater the distance from the reference position, the greater becomes the difference by which the thickness of the cover is larger than the reference thickness.