Background: Low antigravity muscle mass is strongly associated with poor prognosis in patients with chronic obstructive pulmonary disease (COPD). However, the significance of longitudinal changes in antigravity muscle mass remains unclear in patients with COPD. Objectives: The aims of this study were to investigate the factors associated with the longitudinal loss of antigravity muscles and whether the accelerated loss of these muscles has a negative impact on prognosis. Methods: This study was part of a prospective observational study at Kyoto University. We enrolled stable male patients with COPD who underwent longitudinal quantitative CT analysis of the cross-sectional area of the erector spinae muscles (ESM<sub>CSA</sub>) at an interval of 3 years. The associations between the rate of change in ESM<sub>CSA</sub> (%ΔESM) and clinical parameters, such as anthropometry, symptoms, lung function, exacerbation frequency, and all-cause mortality, were investigated. Results: In total, 102 stable male COPD patients were successfully evaluated in this study (71.3 ± 8.3 years, GOLD stage I/II/III/IV = 20/47/28/7 patients). ESM<sub>CSA</sub> significantly decreased from 30.53 to 28.98 cm<sup>2</sup> (p < 0.0001) in 3 years, and the mean %ΔESM was 5.21 ± 7.24%. The rate of survival during the observation period was 85.3% (87/102). Patients with an accelerated decline in ESM<sub>CSA</sub> (n = 31; more than double the mean rate of decline) had a significantly higher frequency of moderate-to-severe exacerbations during the interval (p = 0.015). They also had significantly worse survival (p = 0.035 by log-rank test). A multivariate Cox proportional hazard model showed that lower ESM<sub>CSA</sub> and greater %ΔESM decline were independently and significantly associated with mortality. Conclusions: Frequent exacerbations were related to the loss of antigravity muscles in COPD patients. The accelerated loss of antigravity muscles was associated with a poor prognosis.