Jatropha has been drawing attention as a raw material for biodiesel fuel. For the sustainable production of Jatropha , it is preferable to recover and circulate the fertilizer components contained in seed oil cake to the land used for cultivation. Jatropha seed oil cake contains phorbol esters, a carcinogenic promoter. It is important for the workerʼs health to reduce phorbol esters in the reuse of seed oil cake. Therefore, the purpose of this research was to determine the behavior of phorbol esters in composting Jatropha seed oil cake. Jatropha seed oil cake was composted and the change in phorbol ester concentration in the compost was measured. The concentration of phorbol esters in Jatropha seed oil cake decreased due to composting to less than 10％ of the initial value after 60 days. Furthermore, the decomposition of phorbol esters used for the seeding of compost by lipase-producing bacteria in the compost was investigated. The phorbol ester concentration decreased due to the isolated lipase-producing bacteria.