Precipitation of airborne microplastics (MPs) by rainfall is one of the major transport pathways of MPs from land-to-marine. While most studies examining wet precipitation of MPs collect surface runoffs, direct investigations of MPs in rainwater are hardly reported. In this study, high-frequency and direct rainwater sampling methodology considering the first-flush effect was demonstrated. The variations in MP abundance were evaluated by the inlet size of rainwater collector, time, and duration of sampling. As a result, a stable abundance of MPs was obtained when samplings were conducted at the same time and duration even with different collectors. On the other hand, the abundance increased as much as 4.5 times in samples collected at different times due to the first-flush effect of rainfall. Thus, our methodology that presents MPs concentration versus time curves based on high-frequency sampling would be helpful for easy comparison between similar rainfall studies. Display omitted •Collector size- and time-dependent direct sampling of rainwater was conducted.•Microplastics (MPs) abundance in rainwater was observed by μFT-IR microscopy.•Sampling duration was more significant than sampling volume for comparison purposes.•The abundance of MPs in rainwater was time-dependent due to the first-flush effect.•More MPs existed in the rainwater during earlier stages of rainfall.