A program conversion device generates a program by obfuscating an original program, and generates and encrypts conversion parameters for inverse conversion of the obfuscated program. The program conversion device distributes the obfuscated program together with the encrypted conversion parameters. To execute the obfuscated program, a device having a high security level decrypts the encrypted parameters by using a decryption key stored in advance, applies inverse conversion to the obfuscated program by using the decrypted conversion parameters, and executes the program resulting from the inverse conversion. A device having a low security level executes the obfuscated program without any inverse conversion.