1. A traffic output suppression apparatus for preventing congestion in an asynchronous transfer mode network having a network node, comprising: input cell classification processing means for detecting virtual connection identifiers and cell loss priority information from an input cell stream and classifying the input cell stream into real-time process type cells and process-after-standby type cells according to service characteristics required by virtual connections corresponding to the input cell stream; real-time process type cell processing means for selectively discarding or passing the real-time process type cells from said input cell classification processing means in response to the cell loss priority information from said input cell classification processing means and network node state information received from the network node; network node state processing means for generating a storage command and an extraction command as control signals for the processing of the process-after-standby type cells from said input cell classification processing means in response to the network node state information; process-after-standby type cell processing means for processing the process-after-standby type cells from said input cell classification processing means in response to the storage command the extraction command from said network node state processing means, said process-after-standby type cell processing means outputting directly or storing the process-after-standby type cells from said input cell classification processing means if the network node state is a normal state, and storing the process-after-standby type cells from said input cell classification processing means without outputting them if the network node state is a quasi-congestion state or a congestion state; and cell output processing means for transferring the output cells from said real-time process type cell processing means or the output cells from said process-after-standby type cell processing means in a first-in-first-out manner.