Stable isotope ratios of carbon and nitrogen have been used as an indicator for food sources and trophic levels, respectively. Despite its excellences in the study of food webs, its use would be limiting unless multi-source mixing models are applied. When the entities from the investigation site show weak δ¹³C variations like in a temperate paddy field, it is hard to discriminate food sources or preferred food of predators. This would make it difficult to apply the multi-source mixing models. A simple prey-predator system (fruit fly with different mixtures of food sources and spider) was evaluated to investigate the effects of the carbon and nitrogen stable isotope ratios of a prey on a predator, which would provide an analytical insight for stable isotope ratio estimate data of rice fields. We served media made of banana (B) and/or maize powder (M) as food sources for fruit fly and ¹⁵N-Urea (H) was added selectively. Fruit flies, Drosophila melanogaster, from four kinds of media were served in different ratios to field collected Pirata subpiraticus which is a dominant spider in a paddy field. There were no significant differences in the δ¹³C between spiders which fed different ratios of prey from non-enriched media, although flies showed significant differences (F=22.03, p<0.0001). The δ¹⁵N value of spiders significantly differed between prey types (F=125.37, p<0.0001). Spiders which fed heterogeneous combinations of preys from ¹⁵N enriched and normal medium showed intermediate δ¹⁵N value of between values from spiders which fed only ¹⁵N enriched and non-enriched prey.