Utterances of five Japanese infants and their parents were recorded longitudinally and used to develop an infant speech database. This database was used to analyze the voiced-segment ratio to investigate the developmental changes in utterances produced by infants and parents. The voiced-segment ratio is the ratio of the summed duration of a voiced segment to the total utterance duration. The analyses showed that the ratio tends to increase in an infant's utterances before the infant starts to produce 2-word sentences. The analyses also showed that, at the same stage, the ratio is higher in parents' infant-directed speech than in parents' adult-directed speech. These results suggest that the voiced-segment ratio reflects the development of an infant's utterance ability, and that a higher voiced-segment ratio is one of the characteristics of infant-directed speech.