Provided are an apparatus and a method for downloading Subscriber Identity Module (SIM) data in a mobile communication system, which includes an authentication server that stores authentication information and a list of registered networks; a service provider server that stores SIM data and transmits the SIM data in response to a request for the SIM data; and a mobile communication terminal that receives user authentication from the authenticating server, checks the list of the networks, allows a network, which is to be accessed, to be selected from among the list of the networks, and accesses the service provider server in the selected network so as to download the SIM data.