RibotestMycoplasma (Ribotest™), a rapid antigen detection assay for ribosomal protein L7/L12 for the diagnosis of Mycoplasma pneumoniae infection, has become available in Japan. However, the clinical utility of Ribotest remains controversial. We enrolled 1,140 children admitted to our hospital between January 2014 and March 2016 due to community-acquired pneumonia. We prospectively obtained two throat swabs during the acute phase; DNA detection using a loop-mediated isothermal amplification (LAMP) assay and antigen detection using Ribotest were performed for each sample. We also collected paired serum samples during the acute and convalescent phases for determining M. pneumoniae antibody titers using the particle agglutination test. M. pneumoniae pneumonia was diagnosed through either a positive LAMP assay or a 4-fold increase in antibody titers. Overall, 237 children (21%) were diagnosed with M. pneumoniae pneumonia. We evaluated the utility of Ribotest both in the non-epidemic period (January 2014–July 2015) and the epidemic period (August 2015–March 2016). Sensitivity of Ribotest for M. pneumoniae pneumonia was 23% in the non-epidemic period and 22% in the epidemic period, respectively. When serology was used as the standard, sensitivity of Ribotest was 25% in the non-epidemic period and 22% in the epidemic period, significantly lower than those of the LAMP assay (80% and 91%, respectively). Ribotest yielded false-positive results in 16 cases in the non-epidemic period and in 6 cases in the epidemic period. Thus, positive predictive values of Ribotest were significantly lower in the non-epidemic period (50%) than in the epidemic period (86%). Multivariate analysis showed that a shorter duration of fever before sampling (OR = 1.7) and a higher incidence of co-infection with other pathogens (OR = 29.4) were observed in children showing false-positive results of Ribotest. Thus, we conclude that Ribotest is unsuitable for rapid diagnosis of pediatric M. pneumoniae pneumonia.