A filament lamp that allows independent control of the state of luminescence of multiple filaments and that reliably prevents the occurrence of unwanted discharge between adjacent portions of neighboring filaments, even when a high voltage is injected into the filaments to achieve a desired irradiation distribution, and light-irradiation-type heat treatment device that can heat the article to be treated uniformly. The filament lamp has multiple filament assemblies, each having a filament and respective leads arrangement sequentially within a light emitting bulb, in the axial direction of the light emitting bulb. With alternating current power supplied to each filament independently, the current will be supplied with the same phase and mutually adjacent terminals of neighboring filament assemblies will have the same potential, and with direct current power supplied to each filament independently, adjacent terminals of neighboring filament assemblies will be of the same polarity. The light-irradiation-type heat treatment device uses multiple filament lamps of this type.