1. A program execution control device for causing a processor to execute a program composed of one or more sets of bytecodes including a bytecode for invoking a bytecode set, comprising: a judging unit operable, for each execution of an invocation bytecode during execution of the program, to judge whether a bytecode set targeted for invocation is already compiled to native code specific to the processor; a first unit operable, when the target bytecode set is judged to be uncompiled, to instruct the processor so that the target bytecode set is sequentially interpreted and executed, and to issue a request to compile the target bytecode set to native code; a second unit operable, when the target bytecode set is judged to be compiled, to instruct the processor to execute native code resulting from the compilation; and a third unit operable to instruct the processor to compile a bytecode set indicated by a compilation request issued by said first unit, in parallel with the bytecode interpretation and execution by said first unit as well as with the native code execution by said second unit.