The formation of default boundaries is one of the most important factors to characterize structural models and to explain observed credit spreads. According to empirical studies, we need to take into account market-wide indicators that have exploratory powers for observed credit spreads. Other literature shows that historical credit spreads display countercyclical movement. Since the attribute of default boundaries determines that of credit spreads, this paper introduces a simple structural model where a default boundary of the firm is countercyclical. The purpose of the paper is to capture observed credit spreads through the default boundary affected by the stock market performance as a proxy of a market-wide factor. Since this model has a simple default boundary, we can easily obtain risky bond prices and credit spreads implied in the model. Compared with the existing structural model that considers stock market performance, this paper shows that this countercyclical default boundary model can generate credit spreads that are much closer to observed credit spreads.