A tamper detection device detects tampering with a program loaded to memory, at high speed and without compromising the safety. Prior to loading of a program, a dividing-size determining unit 12 determines a block size based on random number information, a dividing unit 13 divides the program by the block size into data blocks, and a first conversion unit 14 converts, by conducting a logical operation, the data blocks into intermediate authentication data no greater than the block size, and a second conversion unit 15 conducts a second conversion on the intermediate authentication data to generate authentication data. The authentication data and the block size are stored. After the program loading, a program resulting from the loading is divided by the block size, followed by the first and second conversions to generate comparative data. The comparative data is compared with the authentication data to detect tampering of the loaded program.
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