Auxiliary flame retardant, and flame-retardant vinyl chloride resin composition

Provided is an auxiliary flame retardant for imparting high flame-retardant properties to a vinyl chloride resin when used concurrently with a small amount of an antimony trioxide. An auxiliary flame retardant is created by combining, at a mass ratio of 1:1 to 4:1, a 3.5 H 2 O zinc borate having crystal water, and at least one inorganic microparticle having an average particle size of less than 12 μm and selected from among talc, bentonite, dolomite hydroxide and sedimentary rocks. This auxiliary flame retardant is added into a vinyl chloride resin composition comprising a vinyl chloride resin, an antimony trioxide, and a plasticizer such that the amount of the flame retardant added is 1 to 3.1 mass% relative to the total amount. As a consequence, it is possible to attain high flame-retardant properties.