One of the recently widely accepted theories regarding auditor changes focuses on the concept of auditor credibility. According to this theory, larger accounting firms provide more credible audit services than smaller accounting firms. Until recently, many accounting researchers have attempted to empirically test the hypothesis derived from this theory (the so-called qualitative differentiation of audit services) (e.g., Shockley and Holt, 1983; Nichols and Smith, 1983; Francis, 1984; Palmrose, 1986; Ettredge, Shane and Smith, 1988). However, their research findings were insufficient to conclusively support the theory. The purpose of this study is to provide additional empirical evidence on this research issue. More specifically, this study seeks to empirically test whether differences in audit service quality as perceived by investors vary according to the size of the audit firm. For the empirical test, we assume that the corporate audit industry can be simply dichotomized (i.e., Big Eight accounting firms versus non-Big Eight firms) and test whether investors assign differentiated credibility between Big Eight firms and non-Big Eight firms in the capital market. More specifically, this study tests whether a significant change (increase or decrease) occurs in the relationship between accounting earnings and stock returns when a firm changes its auditor from a Big Eight firm (BE) to a non-Big Eight firm (NBE), or vice versa. The results of the empirical analysis indicate that the association between accounting earnings and stock returns (measured by the earnings response coefficient and the correlation coefficient between the two variables) increased (decreased) in a statistically significant manner when a firm changed its auditor from a non-Big Eight firm (Big Eight firm) to a Big Eight firm (non-Big Eight firm). These results imply support for the "Differential Audit Quality Hypothesis" and may provide an additional explanation for the phenomenon of why auditor changes predominantly occur from smaller firms (non-Big Eight) to larger firms (Big Eight).