1. A parallel execution processor comprising: a plurality of processing elements; an obtaining unit operable to obtain (i) a piece of group number information indicating how many groups the processing elements should be formed into and (ii) an instruction sequence including one or more instructions; a decoding unit operable to decode the obtained instruction sequence; a group forming unit operable to form the processing elements into as many groups as indicated by the piece of group number information; and an execution controlling unit operable to assign part or all of the instructions included in the decoded instruction sequence to the groups, so that one group receives one instruction, and control the processing elements so that (i) the instructions received by the groups are executed in parallel, and (ii) in each group, all processing elements in the group are employed in parallel for the execution of the received instruction.