Terahertz (THz) electromagnetic waves hold great potentialities to be utilized for applications such as large-capacity, ultrahigh-speed wireless communications beyond 5G as well as non-destructive inspection of hazardous substances. Plasmonic THz devices based on two-dimensional (2D) plasmons in transistor channels have been studied as potential candidates of on-chip, room-temperature operating, high-speed THz detectors [1] . In particular, graphene Dirac plasmons are promising to break through the limit on operating frequency and sensitivity performance in existing technology [2] . In this paper, we propose our original graphene-based device structure and demonstrate its capability of fast, sensitive THz detection.