Any packet loss is detected very quickly by means of only a series of sequence number in a multi-path environment where a transmitter and a receiver are connected to each other by way of a plurality of networks when no inversion of sequence arises in any of the networks. A communication apparatus includes a plurality of sequence buffers arranged at each network to accumulate packets until a sequence acknowledgement and an absence detecting section adapted to determine the occurrence of an absence of a packet when one or more packets are accumulated in all the sequence buffers. With this arrangement, the absence detecting section of the receiver monitors the packets staying in the sequence guaranteeing buffer arranged in each of the network, paying attention to the characteristic that packets are stored in the sequence buffers of all the networks when a packet loss takes place.