Anabaena sensory rhodopsin (ASR) is well-known as the only retinal protein that achieves the photochromic reaction between the all-trans form (AT-ASR) and the 13-cis form (C-ASR). Although it is known that the structure of the hydrogen-bonding network of ASR is pH-dependent, it is so far unclear how pH affects the photoreaction of ASR. We investigated the pH dependence of the photoreaction of AT-ASR by means of time-resolved absorption spectroscopy and found it to be extremely dependent on pH. At pH 7 and 9, not only the L intermediate but also the K intermediate consisted of two decay components. The formation ratios of the two distinct L intermediates L(fast):L(slow) at pH 7 and 9 were different from each other, although the K(fast):K(slow) ratio was pH-independent. The photoreaction at pH 5 was entirely different from that at pH 7 and 9. Two K intermediates existed, but their formation ratio and lifetimes were different at pH 7 and 9. Moreover, only one L intermediate exists, with a longer lifetime relative to pH 7 and 9. The final product of the photoreaction of AT-ASR was C-ASR at all pH values. Finally, we successfully determined the pH-related photoreaction pathway of AT-ASR.