We study the stability of Kaluza--Klein (KK) modes in five-dimensional N=1 supersymmetric QCD compactified on S1 using the D3-brane probe realization. We discover a phenomenon in which the quark KK mode with KK number n = 1 decays while other KK modes are stable. This is contrary to the ordinary assumption that the n = 1 state is the most stable among quark KK modes. In addition, we show that a massive gauge singlet state carrying the KK number exists and is stable. This thus represents a possibility as the identity of dark matter.