A flame-retardant artificial leather sheet substrate was granted a patent. The nonwoven fabric sheet is made of three-dimensionally entangled superfine fibers of at most 0.5 dtex in fineness. A polymer elastomer fills in the nonwoven fabric and the superfine fibers comprise an organophosphorus component-copolymerized polyester. The polymer elastomer satisfies at least one of the following: (1) the polymer elastomer contains a metal hydroxide; (2) the polymer elastomer is copolymerized with an organophosphorus component and the flame retardant artificial leather sheet substrate has a fire spreading speed of not more than 100 mm/min. as measured in the combustion test method of JIS D1201.