Spontaneous spiral formation has been observed in heterogeneous excitable media, and identified unidirectional paths as the origin of the spiral cores. Our numerical results showed unidirectional propagation always appeared at the spiral core as reported on the previous paper (Kinoshita et al., 2013). To clarify the cause of unidirectional behavior, we focused on the inhibitor concentration around the path and found a spatial asymmetric profile. We concluded that the asymmetric inhibitor profile, which was generated by a simple asymmetric construction of the suppressed blocks, induced spiral wave on the heterogeneous reaction field. (C) 2014 Elsevier B.V. All rights reserved.