Radio wave absorbing composition and radio wave absorber

A radio wave absorber,

• which comprises a radio wave absorber layer comprising:
• a magnetic powder;
• and
• a binder,
• wherein the magnetic powder is a powder of a substitution-type hexagonal ferrite subjected to surface treatment with a surface treatment agent,
• the surface treatment agent is a silicon-based compound, and
• the binder is an olefin-based resin, said olefin-based resin being one or more resins selected from the group consisting of polymethylpentene, a cycloolefin resin, and an olefin-based resin containing a maleic acid anhydride unit, and
• which comprises the magnetic powder in an amount of 50% by mass or more and 75% by mass or less, with respect to the total mass (100% by mass) of the magnetic powder and the resin, and
• which does not comprise a metal layer, the metal layer being a layer containing a metal and substantially reflecting radio waves, wherein “substantially reflecting radio waves” means reflecting 90% or more of incident radio waves in a case where the radio waves are incident on the radio wave absorber layer in a state where the metal layer is laminated on the back surface of the radio wave absorber layer.