The objective of the present invention is to disable functionality of an additional-function unit if an unauthorized program has been installed in an information processing device, thereby preventing an unauthorized program from acquiring, in an unauthorized manner, information from the additional-function unit. The present invention is an enable/disable method for an additional-function unit in an information processing device to which the additional-function unit has been added, which has a step for calculating a first directional function value on the basis of data included in a recording medium storing a boot loader and an operating system so as to store the first directional function value at manufacture time into the additional-function unit, a step for calculating a second directional function value on the basis of data included in the recording medium after the information processing device has been started up, and a step for disabling the functionality of the additional-function unit if the first directional function value and the second directional function value are different.