To provide a flame sensor which is resistant to any impact, and which is configurable at low costs.

In a flame sensor equipped with an element which detects an infrared ray to be generated from flame due to fire in a main body, a band pass filter, a silicon filter and a sapphire glass are arranged in this order on the front face of the element so that the window part of the main body can be configured. In this filter configuration to be assembled on the front face of the element which detects the infrared ray, the silicon filter and the band pass filter are arranged in this order with sapphire arranged outside so that the strength of the window part can be made excellent, and that the band pass filter can be made compact, and that costs can be reduced.