A flame resisting agent to be incorporated into an aqueous spinning dope containing chlorine-containing polymer as a main polymer ingredient for improving the flame resistant property of the fiber spun from the spinning dope which includes a stannic oxide hydrate being substantially amorphous with respect to X-ray diffraction and having a bound water in the range of about 0.5 to about 0.7 mole of H.sub.2 O per 1 mole of SnO.sub.2. The flame resisting agent has a superior dispersibility in an aqueous spinning dope containing chlorine-containing polymer as a main polymer ingredient and the aqueous spinning dope in which the flame resisting agent is incorporated has a superior stability, filtering property and spinning property to give a uniform quality of fibers having a durable, superior flame resistant property.