1. A multithread processor for executing a plurality of threads in parallel at every time of issuing an instruction by pipeline processing, comprising: a holding unit to hold information about each of the plurality of threads and a thread identifier to uniquely identify a thread in each piece of the information; a judging unit to judge whether or not a target thread being an object to be synchronized with one thread exists in a same processor by using the thread identifier held by the holding unit; a trap generating unit to let a trap occur when the judging unit judges that the target thread does not exist in the same processor; and an issuing unit to successively input, when the judging unit judges that the target thread exists in the same processor, the target thread as a thread to be executed next, at a time of issuing a succeeding instruction, into the pipeline.