Provided are a method and a device for booting a host embodying a downloadable conditional access system (DCAS), wherein one of a plurality of pre-determined booting modes is decided as a booting mode of the host based on first information indicating whether a host can communicate with a broadcasting service provider and second information indicating whether a software-based security client providing information required for decrypting broadcasting data is installed to a hardware-based security module connected to the host, and the host is booted in the decided booting mode.