This paper presents an improved phoneme-history-dependent (PHD) search algorithm. This method is an optimum algorithm under the assumption that the starting time of a recognized word depends on only a few preceding phonemes (phoneme history). The computational cost and the number of recognition errors can be reduced if the phoneme-history-dependent search uses re-selection of the preceding word and an appropriate length of phoneme histories. These improvements increase the speed of decoding and help to ensure that the resulting word graph has the correct word sequence. In a 65k-word domain-independent Japanese read-speech dictation task and 1000-word spontaneous-speech airline-ticket-reservation task, the improved PHD search was 1.2-1.8 times faster than a traditional word-dependent search under the condition of equal word accuracy. The improved search reduced the number of errors by a maximum of 21% under the condition of equal processing time. The results also show that our search can generate more compact and accurate word graphs than those of the original PHD search method. In addition, we investigated the optimum length of the phoneme history in the search.