FIG. 1 is a side sectional view of a housing portion which has a driver unit according to the present invention; FIGS. 2A and 2B are external views of a headphone apparatus according to the present invention; FIGS. 3A and 3B are a perspective view of a baffle portion and a side sectional view of a back housing, used for the present invention; and FIGS. 4A to 4C are cross-sectional views of a part