1. A multithread execution device comprising: a program memory in which a plurality of programs are stored; an instruction issue unit that issues an instruction of the program retrieved from the program memory; an instruction execution unit that executes the instruction; a target execution speed memory that stores target execution speed of the instruction; an execution speed monitor that monitors an execution speed of the instruction; and a feedback control unit that commands the instruction issue unit to issue the instruction so that the execution speed approximately equal to the target execution speed.