To provide a noise detector which detects noise signals incoming from external apparatus, decides whether the noise signals influence the interior of an electronic apparatus to output a noise detection signal to the electronic apparatus, thereby preventing the electronic apparatus from malfunctioning due to noise.

A noise-deciding part 8 decides that a noise signal influences the process of an external processor 200, if a process status received via a monitor terminal 11a from the processor 200 coincides with a preset noise-deciding processing status and a digital noise signal inputted from an A/D converter 5a exceeds a set value stored in a register 6a. A ROM 7 stores the received process status in connection with a corresponding signal class under detection, and the noise detection signal is fed to an output circuit 9 to output a reset signal to the processor 200 from the output circuit 9, thereby changing it to a fail/safe operation.