Underreporting is a problem in dietary surveys, and data on Japanese individuals with obesity are lacking. In addition, in dietary surveys of individuals with obesity, underreporting and extreme energy restrictive practices for short periods of time have been reported, and blood total ketone levels (ketone bodies) may be able to distinguish between these factors. The present study aimed to examine the relationship between underreporting [energy intake (EI)/basal metabolic rate estimate (BMR)] and ketone bodies in obese Japanese women. The participants included 91 women with obesity aged 47±9 years with a body mass index (BMI) of 29.8±3.9 kg/m² who met the exclusion criteria out of 164 individuals who participated in an institutional cohort study baseline survey between September 2006 and September 2015. The current study defined the relationship between EI/BMR, BMI and the participants' ketone body levels. EI/BMR <1.35 and ketone body level <1.0 mmol/l was defined as underreporters, while EI/BMR <1.35 and ketone body level ≥1.0 mmol/l was defined as energy-restricted reporters based on previous research. The EI/BMR of the participants was 1.44±0.32, and 25.3% had an abnormally high level of ketone bodies. Multiple regression analysis indicated that ketone bodies were explanatory variables for EI/BMR. Analysis using EI/BMR and ketone bodies estimated that 26.4% were underreporters and 12.1% were energy-restricted reporters. There were no significant differences in reported energy intake, carbohydrate intake (g/day), and percentage carbohydrate (%) between the underreporters and energy-restricted reporters. In conclusion, low EI/BMR was associated with high ketone body levels in Japanese women with obesity. The combination of EI/BMR and ketone bodies may distinguish between or screen for underreporters and energy-restricted reporters during a dietary survey.