Background: Thinness-oriented body ideals are well described among young Japanese women, but variation in visual and numerical ideals by sex, age, and current BMI remains unclear. Methods: We analysed an unweighted web-panel sample of 27,359 Japanese adolescents and adults aged 15-69 years from the Survey of Body Shape and Health Awareness in Japanese People, a cross-sectional web-based survey. Current and ideal BMI were derived from self-reported height and weight, and visual orientation from ideal versus perceived current silhouette. Adult BMI-stratified analyses included participants aged 20-69 years. Findings: Among 27,359 participants (10,094 men and 17,265 women), relative thinness-oriented indicators increased with current BMI in both sexes. Among normal-weight women, 74.7-83.7% selected a smaller ideal silhouette and 79.2-91.2% reported an ideal BMI lower than current BMI. These indicators were also present among underweight women aged 20-29 years (44.9% and 43.4%, respectively), but declined with age. Most underweight men reported weight-gain-oriented ideals. Fine stratification showed that thinness-oriented ideals among women were common in low-normal and mid-normal BMI strata. Interpretation: Body-size ideals differed by sex, age, and current BMI. Thinness-oriented ideals among women extended beyond obesity and young adulthood, whereas underweight men generally preferred a larger body. Longitudinal and behavioural studies are needed. Funding: Cabinet Office of Japan Strategic Innovation Promotion Program (SIP), "Inclusive Community Platform" (grant no. SIPCP23C02).