In a high mobility environment, single-carrier (SC) space-frequency block coding (SFBC) diversity achieves better bit error rate (BER) performance than SC space-time block coding (STBC) diversity. However, in a strong frequency-selective fading channel, the BER performance degrades due to the orthogonality distortion of SFBC codeword. In this paper, we propose a frequency-domain equalization (called robust FDE) suitable for SC-SFBC diversity in a frequency-selective fading. The robust FDE weights are jointly optimized based on minimum mean square error (MMSE) criterion taking into account channel variation within a SFBC codeword caused by frequency-selective fading. It is shown by computer simulation that proposed robust FDE always achieves BER performance superior to conventional FDE whose weights are determined without considering the channel frequency variation within a SFBC codeword.