1. A computer-readable storage medium storing a service process analysis program for analyzing service process status in a network system including a plurality of servers, the service process analysis program causing a computer to function as an apparatus comprising: (a) a message collector that collects messages exchanged in the network system; (b) a message parser that parses the collected messages to determine whether each of the collected messages is a request message or a response message, and occurrence time, source, and destination of the each of the collected messages, and stores records of the parsed messages in a message record memory, each of the records including results of the parsing; (c) a definite transaction finder that identifies definite transactions by extracting, from the records stored in the message record memory, a combination of messages having unique and definite call dependency; (d) a transaction candidate builder that selects each of the messages included in neither the definite transactions nor previously estimated transactions, extracts a plurality of combinations of messages including the selected message and the message satisfying a predetermined requirement concerning the call dependency to produce one or more transaction candidates from the plurality of combinations of messages; (e) a candidate evaluator that evaluates likelihood of each of the transaction candidates, based on how many transactions of the same class have been identified as the definite transactions or estimated transactions; (f) a transaction estimator that produces another estimated transaction by selecting one of the transaction candidates based on the likelihood thereof; and (g) a result output unit that outputs the definite transactions and estimated transactions as an analysis result.