1. A method comprising: extracting a feature indicative of a property of a vocal tract of a speaker from each of training source speech and training target speech; defining sub-feature units with respect to the feature for both the training source speech and the training target speech to generate training source speech sub-feature units and training target speech sub-feature units, respectively; and performing voice conversion of source speech to target speech based on the conversion of the sub-feature units to corresponding target speech sub-feature units using a conversion model trained with respect to converting the training source speech sub-feature units to the training target speech sub-feature units.