1. A stream data processing method carried out by a stream data processing apparatus for accepting stream data inputted continuously and executing query arithmetic processing on the stream data, wherein: the stream data processing apparatus includes a storage for storing the stream data, a stream control section for controlling the stream data processing apparatus, a query execution section for executing the query arithmetic processing on the stream data, and a scheduler for allocating computer resources of the stream data processing apparatus; the scheduler groups one or more queries defining the query arithmetic processing as a query group and allocates the query group as one unit to a thread of the computer resources to make the query execution section execute the query arithmetic processing on the query group; once data stagnation occurs in a given query group, the scheduler reads at least one of input flow rate information and latency information of each query of the given query group from the storage, and calculates an estimated load value of the query; the scheduler divides queries belonging to the given query group into a plurality of query groups so that the sum of estimated load values of queries of one query group can be substantially equalized to the sum of estimated load values of queries of another query group; and the scheduler gives an instruction to the stream control section to reallocate the plurality of divided query groups to different threads of the computer resources respectively.