1. A program execution device comprising: first storage means for storing data obtained by compressing a program code constituted by an execution code that is composed of process data used for an execution process and a command representing the execution process which are arranged in order; second storage means for temporarily storing the process data; type determination means for reading the data obtained by compressing the program code from the first storage means by a predetermined data amount each to determine whether or not the data contains the command, and storing the process data in the data in the second storage means when it is determined that the data does not contain the command; code extension means for generating an execution code on the basis of the process data stored in the second storage means and the command when the type determination means determines that the data contains the command; and execution means for sequentially performing the execution process of the program code on the basis of the execution code generated by the code extension means, wherein the process data is piled in a stack, and the execution means performs the execution process on the basis of the program code described in a stack type language for the command to act on the stack to perform the execution process.