We study preferential Bayesian optimization (BO) where reliable feedback is limited to pairwise comparison. An important challenge in preferential BO, which uses the Gaussian process (GP) model to represent preference structure, is that the posterior distribution is computationally intractable. Existing preferential BO methods either suffer from poor posterior approximation ignoring the skewness or require computationally expensive approximation for the exact posterior represented as a skew GP. In this work, we develop a simple and computationally efficient preferential BO algorithm while keeping the superior optimization performance. The basic idea is to use a posterior additionally conditioned by a random sample from the original posterior itself, called hallucination, by which we show that a usual GP-based acquisition function can be used while reflecting the skewness of the original posterior. The numerical experiments on the various benchmark problems demonstrate the effectiveness of the proposed method.