In a transmitting device, a receiving device, and an operation method thereof according to the present invention, in relation to a simultaneous link transmission of divided data through a plurality of heterogeneous networks for one session, particular partial data to be transmitted or received through a network vulnerable to a security problem among divided partial data is forcedly divided and is then transmitted through two or more networks or a partial data packet included in particular partial data is forcedly re-divided and is then transmitted through two or more networks, so as to effectively conceal the service and thereby prevent other people or another party from recognizing the service through copying or intercepting of data packets.