The potential of pharmaceuticals and personal‐care products to alter the behavior of aquatic organisms is a growing concern. To assess the actual effect of these substances on aquatic organisms, a simple but effective behavioral test is required. In this study, we devised a simple behavioral test (peek‐a‐boo test) to assess the effect of anxiolytics on the behavior of a model fish (medaka, Oryzias latipes). In the peek‐a‐boo test, we investigated the response of medaka to an image of a predator fish (donko fish, Odontobutis obscura). The test revealed that the time taken for test medaka exposed to diazepam (0.8, 4, 20, or 100 µg/L) to approach the image was shorter by a factor of 0.22–0.65 and the time spent in the area close to the image was longer by a factor of 1.8–2.7 than in the solvent control group for all diazepam exposure groups (P < 0.05). Hence, we confirmed that the peek‐a‐boo test could detect changes in medaka behavior caused by diazepam with high sensitivity. The peek‐a‐boo test devised in this study is a simple behavioral test with high sensitivity for fish behavioral alteration.