The sensitivity of the Japanese gravitational-wave detector KAGRA is limited mainly by quantum noise. In order to reduce the quantum noise level, KAGRA employs an output mode-cleaner (OMC), which filters out junk light to clean up the signal and the reference light at the signal extraction port. The proper design of the OMC is a key to achieve the target sensitivity of KAGRA. In this proceeding, we present two results. One is the final result of numerical simulations, from which we determined the optical parameters of the OMC. The other is the latest results of our prototype experiment, the goal of which is to establish the control scheme of the OMC.