To provide a fail safe system which uses a personal computer as a man-machine interface and can secure fail safe property and to provide an information transmission method in the fail safe system.

When a manager inputs control information on a fail safe device, the input part of a personal computer inputs a first value (S1), a fail safe device converts control information into a character code and returns it to the personal computer (S3). When the manager judges the content of transmission/reception data and there is no error, a second value similar to the first value is transmitted to the fail safe device through the personal computer (S7) and the fail safe device decides control information when the second value is matched with the first value (S8).