Peatlands are important ecosystems for understanding global methane (CH4) and greenhouse gas budgets. We evaluated the CH4 emissions from a cool temperate bog in northern Japan and their responses to environmental drivers based on four years of quasi-continuous measurements with the eddy covariance method. The bog acted as a 13 to 19 g C m− 2 yr−1 CH4 source. High annual CH4 emissions outweighed the annual carbon dioxide (CO2) sink strength (-223 to -79 g C m− 2 yr−1), except in one year, resulting in a net greenhouse gas source at the annual timescale, although greater uncertainties in the annual CO2 budget (14%) than the CH4 budget (5%) caused uncertainties in the greenhouse gas budget. The seasonal variations in the CH4 fluxes were well explained by soil temperatures in the deep layers and the leaf area index of vascular plants. Pressure-driven CH4 emissions were measured at synoptic timescales, indicating high contributions of ebullition emissions. The episodic CH4 emissions during two typhoon events accounted for 5.1% of the annual fluxes in 2018. Nighttime CH4 emissions were greater than daytime emissions during the early growing season, possibly due to the hydrodynamic transport associated with soil temperature instability at night. The annual CH4 emissions were high when the dissolved oxygen concentration at 10 cm was low and the annual CO2 sink was great, suggesting the importance of anaerobic conditions and substrate availability, which facilitate CH4 production and restrict CH4 oxidation. The climate conditions in the study years were within the normal range; thus, further long-term measurements covering a wide range of climatic conditions will be required.