A radio wave absorber that can be bonded securely to a flexible cable or a flexible printed circuit board eliminates the need for a release film. A radio wave absorber comprising a thermosetting adhesive sheet 1 that is cured into a flexible material, and a Sendust-containing curable resin composition layer 2 that is cured into a flexible material, wherein the curable resin composition layer is formed on either surface of the thermosetting adhesive sheet. The average particle diameter of Sendust is in the range of 30 to 100 .mu.m. The content ratio of Sendust in the curable resin composition layer is in the range of 50 to 85 wt %.