To provide new fire retardant which does not substantially contain atoms such as halogens, phosphorus, nitrogen and the like and shows an extremely high fire retarding effect and which can employ inexpensive raw materials and which does not damage surface appearance of resins and a fire retarding resin composition prepared by using the fire retardant.

The fire retardant comprises a polymer which comprises silicon, boron and oxygen and which has a skeleton substantially made of a silicon- oxygen linkage and a boron-oxygen linkage and which has an aromatic group in the molecule wherein the polymer has an onset temperature of weight loss of 300°C or more in the thermogravimetric method and does not have a negative peak in the temperature range equal to or above the onset temperature of weight loss in the differential thermal analysis. The fire retarding resin composition contains 0.1-50 pts.wt. of the fire retardant based on 100 pts.wt. of a resin.