PURPOSE: To provide a flame detector which has fail-safe function, being capable of detecting the hormal condition.

CONSTITUTION: When a signal commanding combustion is outputted to a combustion device, a pulse signal outputted by a signaling circuit in a transmitting circuit 12 is transmitted through an electric-optical signal-converting circuit in a transmitting circuit 12, second optical fiber 10, reflecting member 7, first optical fiber 9, optical-electric signal-converting circuit in a receiving circuit 17, and filter circuit, and finally to a judging circuit. Independent of the presence or absence of a flame the judging circuit judges whether the device is normal or not by judgment as reguards the presence of a pulse signal component and a direct-current component of a flame.