To provide a treating oven for imparting flame resistance capable of performing the flame resistance treatment of a precursor fiber strand uniformly, and improving productivity without deteriorating quality.

This treating oven 2 for imparting flame retardance is constituted by having a heat-treating chamber 4 sending hot wind in vertical direction of the precursor fiber strand 6 traveling horizontally in the oven for making the strand as flame resistant, an upper flow passage 8 formed in the upper direction of the heat-treating chamber 4, a lower flow passage 10 formed in the lower direction of the heat-treating chamber and a hot wind-circulating passage 12 communicating the upper flow passage 8 and lower flow passage 10, constituting the lower flow passage with divided in a plurality of flow passages 28, 30 from the hot wind-circulating passage 12 in the heat-treating chamber 4 to an outer wall 16 side opposing to the hot wind-circulating passage 12, and communicating the inside of the heat-treating chamber 4 with the hot wind-circulating passage 12 by these divided flow passages 28, 30.