An Internet group membership protocol (IGMP) network device and a signal processing control method thereof in an Internet protocol (IP) digital broadcasting system are provided. The method includes: when an IGMP packet is received from an IGMP client, determining, at a packet transmission/reception unit, whether or not the received IGMP packet is transmitted via bypass; when it is determined that the IGMP packet received from the IGMP client is transmitted via bypass, copying, at the packet transmission/reception unit, the received IGMP packet and bypass-transmitting an original IGMP packet to an upper IGMP network device; transmitting, at the packet transmission/reception unit, the copied IGMP packet to a host; and receiving, at the host, the IGMP packet from the packet transmission/reception unit, processing the IGMP packet, and simultaneously updating its multicast table and a multicast table of the packet transmission/reception unit.