F18-fluorodeoxyglucose (FDG)–positron emission tomography (PET) along with computed tomography (CT) is a standard method for assessing brown adipose tissue (BAT) activity. We tested the usefulness of near-infrared time-resolved spectroscopy (NIRTRS) as a simple and noninvasive method for evaluating BAT density (BAT-d) by examining the effects of some factors known to influence BAT activity. The total hemoglobin concentration as a parameter of BAT-d was evaluated using NIRTRS in the supraclavicular region in 413 Japanese individuals. The associations were analyzed between BAT-d and sex, age, the percentages of body fat (%BF), visceral fat (VF), and the seasonal ambient temperature (AmT) fluctuations. Age was associated with decreased BAT-d (P < 0.05). There was no sex difference in the BAT-d, except for those in their twenties. Multivariate analyses revealed that %BF and VF were correlated with BAT-d, and the lower AmT (around 4°C or 5°C) for 4 and 6 weeks prior to the measurement day was associated with an increase in the BAT-d. Our NIRTRS results were analogous to those reported with FDG18-PET / CT, indicating the usefulness of NIRTRS. BAT-d might increase during the 4 and 6 weeks after the AmT decreases to lower than 4°C or 5°C.