1. A method for releasing a packet data unit (PDU) session performed by a session management function (SMF) in a network, the method comprising:deciding a single access network, which is an access network to be released in a multi-access packet data unit (MA PDU) session which is already established;
transmitting a first message including first information corresponding to an access type of the decided single access network to a user plane function (UPF);
receiving a response message corresponding to the first message from the UPF; and
transmitting a second message including second information and a PDU session release command message to an access and mobility management function (AMF), wherein the second information indicates the access type of the decided single access network.