This paper describes a novel speech interface function, called speech completion, that helps a user enter a word or phrase by completing (filling in the rest of) a phrase fragment uttered by the user. Although the concept of completion is widely used in text-based interfaces, there have been no reports of completion being effectively applied to speech. By using a filled pause, we enable a user to effortlessly invoke the speech-completion function which helps the user recall uncertain phrases and saves labor when the input phrase is long. When a user hesitates by lengthening a vowel (a filled pause is uttered) during a phrase, our system immediately displays completion candidates whose beginnings acoustically resemble the uttered fragment so that the user can select the correct one. In our experiments with a system that included a filled-pause detector and a speech recognizer capable of listing candidates, the effectiveness of speech completion was confirmed.