This research's aim is to demonstrate the factors involved in the process of improving musical expression in a lesson where actual singing lessons are conducted. This research was conducted by analyzing and examining the data gathered using qualitative research methods and by the author's continuous participant-observation and interviews. As a result, I set five aspects of students' improvement; "Skill of elocution," "Voice resonance," and "Quality of voice resonance" as expression of elocution, and "Constructional aspect of music," "Content aspect of music," as expression in music. I observed two important factors in students' improvement, one is influenced by teachers' guidance and the other is influenced by student's initiative. When I analyzed the data and summarized the result, I concluded that the perception of the teachers' instruction greatly affects the students' improvement. Consequently, verbal communication between teacher and student in a singing lesson, which has not been emphasized in the past, is an efficient method to develop mutual understanding. The teachers' instruction and communication leads to "positive attitude" and significant improvements in students' musical expression.