This paper describes the algorithm, implementation, and evaluation results of a new fault simulator called RFSIM, which is designed for combinational circuits. In order to accelerate fault simulation, two basic principles are introduced, a Detectable Fault Only (DFO) principle and a Candidate Gate Once (CGO) principle. The DFO principle is a dynamic reduction algorithm, which aims at drastically reducing computational complexity by utilizing blocking gate information. The CGO principle is an implementation technique which is utilized to implement the DFO principle effectively. Experimental results show that RFSIM is more than 10 times faster than a conventional concurrent fault simulator, and confirms that the DFO principle contributes to a drastic reduction in the number of faults to be simulated. A fault reduction ratio of around 25 to 1 was achieved in one of the benchmark circuits.