A double-spend attack is one of the major security issues in most blockchain systems, but it is difficult to successfully launch unless an adversary has massive computing power. In this paper, we introduce a new attack model that combines a double-spend attack with a Sybil attack in the Bitcoin network. We present analysis results that a double-spending attacker can make a block propagation delay by conducting Sybil attacks and increase the probability of winning the mining race, thus successfully launching the double-spend attack. We develop the probability of success of this new attack in mathematics forms and analyze this attack model from the perspective of economics. We present the attacker's break-even point in various situations and demonstrate the effect of the proposed attack.