To provide a membrane separation apparatus and a membrane separation method capable of preventing supply of supplied water containing an oxidizing agent to a semi-permeable membrane without supplying an excess amount of a reducing agent.

At the time of carrying out membrane separation by supplying a pretreated supplied liquid to a semi-permeable membrane module equipped with a reverse osmosis membrane and/or a nanofiltration membrane, the membrane separation method involves supplying an oxidizing agent to the supplied liquid during pretreatment or upstream of the pretreatment and supplying a reducing agent downstream of the pretreatment and upstream of the semi-permeable membrane module and the oxidizing agent concentration or the redox potential of the supplied liquid is measured downstream of the pretreatment and upstream of the semi-permeable membrane module to determine the necessity of the reducing agent supply and the oxidizing agent concentration or the redox potential of the supplied liquid is measured downstream of the reducing agent supply and upstream of the semi-permeable membrane module to determine the supply amount of the reducing agent.