The concept of a traveling-wave direct energy converter (TWDEC) to recover the energy of 14.7-MeV protons produced by D-3He fusion has been proposed. In TWDEC, the protons are velocity modulated in the modulator to form a bunched beam and then decelerated by the traveling wave excited in the decelerator. A proof-of-principle experiment of TWDEC is performed using a low-energy ion beam. Characteristics of velocity modulation and bunching are measured and compared with theory. It is demonstrated that the beam is decelerated by the traveling wave when the phase velocity and the phase of the wave are adjusted with respect to those of the bunched beam.