In this paper, we propose an adaptive single-carrier (SC) transmission suitable for near maximum likelihood (ML) block signal detection using QR decomposition and M-algorithm (QRM-MLBD). QRM-MLBD can significantly improve the bit error rate (BER) performance of SC block transmission. However, in order to achieve a close-to-ML performance, the use of a fairly large number M of surviving paths in the M-algorithm is required and hence its computational complexity is still high. In this paper, to reduce the required number of surviving paths, we introduce an unequal bit loading in an SC transmission block. In QRM-MLBD, the signal-to-noise power ratio (SNR) degrades for symbols to be detected at early stages in the M-algorithm. By noting this fact, we propose to load less number of bits (i.e., lower modulation level) on symbols close to the end of block. We show that the introduction of an unequal bit loading to SC block transmission using QRM-MLBD can achieve almost the same BER performance as the conventional QRM-MLBD while significantly reducing the required number M of surviving path in the M-algorithm.