The purpose of this study is to clarify the effects of brightness and loudness of the background music on interpersonal communications in a cafe. An investigation to observe the conversational behavior of subjects, and an experiment to estimate suitability of lighting and sound environments for various behaviors were carried out under four different conditions. The principle results are summarized as follows. 1. Women tended to have more frequent eye contact than men under every condition. 2. Men tended to talk considerably under low illumination, and women tended to talk more under conditions of low sound volume. 3. Men tended to bend forward under high illumination, and women tended to bend forward under low illumination. 4. There were combined effects of illumination and music loudness on the evaluation of usage behaviors. For example, studying was appropriate in bright and quiet condition.