In the present study, ten undergraduate interpreting students and those of two professional interpreters were compared. It was found that decreasing accuracy of interpretation of the following sentence was identified as one of the student problems. Also, it was examined that there was an interpreting unit where the participants who used natural Japanese expressions in the proceeding sentence but with fillers, resulting in omitting or substituting interpretations in the following sentence. It is suggested that some of the student participants were not able to de-verbalize the English word in the source speech, and resulting eventually in the omission and substitution. Therefore, it is recommended that students should be taught a de-verbalization process (Selescovith,1989) as part of interpreting theory.