Microplastic ingestion negatively affects benthic organisms by physical effects of microplastic ingestion, and by chemical effects from persistent organic pollutants (POPs) which accumulate microplastics on their surface. In this study, we tested effects of microplastics on the lugworm Abarenicola pacifica, a dominant deposit-feeder in intertidal flats of Akkeshi-ko estuary, eastren Hokkaido, Japan. We conducted the experiment for 12 weeks by exposing the lugworm to different microplastic contents with/without fluoranthene as a model POP. The microplastics negatively affected feeding rate of the lugworm within a week. However it did not affect their feeding rates in 6–12 weeks nor the lugworm weight. Their burial time and hypoxia tolerance were positively affected by the microplastics. For fluoranthene, the positive effects were found for the feeding rate within a week and for the hypoxia tolerance. Effects were not significant for the feeding rates in 6–12 weeks, the lugworm weight nor the burial time. Only the feeding rate within a week showed negative effect of microplastic, which may indicate the high tolerance of the lugworm to microplastic impacts.