To improve the quality of concept testing, it is necessary to appropriately design it from the perspectives of who (extraction of respondents), what (information to present), and how (information presentation method). Here, we focused on who factors. Since concept testing requires survey respondents to understand the target concept, existing literature has advocated hiring people familiar with the target product. However, when developing new products, the concept should be innovative. There is little need for concept testing for products already familiar to consumers. Therefore, this study introduced innovation diffusion in selecting respondents for concept testing. We distinguished between early adopters (innovators, early adopters, and early majority) and late adopters (late majority and laggards). We derived the following hypotheses: H1: Estimating market share based on concept testing responses is more accurate for early adopters than late adopters. H2: The relationship between concept testing responses and actual purchase behavior is stronger for early adopters than for late adopters. We conducted concept testing on washing machines (drum and vertical types) in Japan online with 1,200 people and compared the results with actual sales data. Therefore, H1 and H2 were supported. This method, identifying appropriate respondents with a single question, would be useful for businesses as it does not increase survey costs or labor for respondents.