Provided is a multithreaded processor that can accurately estimate processing time necessary for each thread, and a multithreaded processor that simultaneously executes instruction streams, the multithreaded processor including: a computing unit group that executes instructions; an instruction scheduler that groups the instructions into groups for each of the instruction streams, the instructions being included in the each of instruction streams, and each of the groups being made up of instructions among the instructions to be simultaneously issued to the computing units; an instruction buffer which holds the instructions for each of the groups grouped by the instruction scheduler, the instructions being included in the each of instruction streams; and an issued instruction determining unit that reads the instructions for each of the groups from the instruction buffer in each of execution cycles of the multithreaded processor, and that issues the read instructions to the computing unit group.