The percent modern carbon (pMC) values of natural rubbers from certain global areas as biomass resources and foods in Japan were confirmed by accelerator mass spectrometry (AMS). The pMC value of biomass harvested in 2014 and 2015 was determined to be 101%. The optimal conditions for AMS pretreatment procedures were examined. Eleven model rubber compounds were prepared to determine biobased contents. The biobased contents of the model rubber compounds were calculated using their formulations and measured by carbon-14 concentration (equivalent to percent modern carbon) using AMS. The biobased carbon contents of the rubber compounds were estimated based on ISO 19984-1 by the ratio of biobased carbon mass to total carbon mass in the sample material and measured based on ISO 19984-2 using AMS for the sample itself. Their biobased mass contents were calculated based on ISO 19984-1 as the ratio of biobased ingredient mass to total sample mass and measured based on ISO 19984-3 by chemical analysis and AMS for separated samples of rubber, solvent extract, and inorganics including carbon black. The reproducibility of the biobased contents was determined by round robin test (RRT) in the ISO technical committee 45 “Rubber and Rubber Products” during preparation, documentation, and discussion of the ISO 19984 series. Nine AMS laboratories and one chemical evaluation laboratory, located in four countries, participated in this RRT. These methods for determining biobased contents were confirmed to have high reproducibility. The biobased carbon content and the biobased mass content of commercial rubber products, such as tire parts, were successfully measured based on ISO 19984-2 and 3.