The purpose of this study was to investigate the effects of enhancement of systemic circulation due to repetitive hot water bathing on work-related chronic shoulder-neck muscle pain. Eight female office workers reporting chronic shoulder-neck muscle pain used a bath additive, KYN, while they took baths at home for 2 weeks and then took baths without KYN for another 2 weeks. KYN contained CO2 generators and 3-octylphthalide that produced final concentrations of 100 ppm and 3 ppm in 150 L bath water, respectively. The effects of KYN were evaluated during and after the use of KYN using a visual analog scale (VAS), fatigue scores, muscle hardness, near-infrared spectroscopy (NIRS), and the pressure pain threshold (PPT). During the use of KYN, VAS significantly decreased (p<0.05), as did fatigue score III expressing "sense of physical discomfort" (p<0.01) and muscle hardness (p<0.01). These indices expressed increases during the next 2 weeks of hot water baths without KYN. In conclusion, the effects of repetitive use of KYN were clarified by subjective and physiological parameters on work-related chronic shoulder-neck pain, and they could be ascribed to cumulative immediate enhancement of systemic circulation. Further studies are necessary to clarify the correlations among parameters and underlying mechanisms.