1. An image forming apparatus having a first execution environment that can interpret and execute a first-format instruction for controlling said image forming apparatus, and a second execution environment that can interpret and execute a second-format instruction defined by a format that does not depend on a type of said image forming apparatus, realized in the first execution environment, said image forming apparatus comprising: control means for controlling processing of starting execution of the first-format instruction from the second execution environment, using a second-format predetermined instruction; recognition means for recognizing, when interpreting the second-format instruction by executing application software in the second execution environment, whether or not an instruction string to be executed is the second-format predetermined instruction; and control means for controlling said image forming apparatus by starting and executing the first-format instruction via the second-format predetermined instruction when said recognition means has recognized that the instruction to be executed is the second-format predetermined instruction, and for controlling said image forming apparatus so as to execute the second-format predetermined instruction in the second execution environment when said recognition means has recognized that the instruction to be executed is not the second-format predetermined instruction, wherein said first-format instruction and said second-format instruction are encoded in a byte code form.