The crustally-guided shear wave, Lg, is typically the strongest phase at regional distances. Lg phases are analyzed often for estimation of magnitudes of regional events. The variation of Lg in regions with both continental and oceanic environments has been rarely investigated. We investigate Lg attenuation in a plate-margin area around Korea and Japan that is encompassed by seas and oceans. The mean quality factor of Lg at 1 Hz (Q0) is 498. The geometrical-spreading exponent term is estimated as 0.75. These observations characterize the study region as a high attenuation region compared to typical intraplate continental regions. Low-Q regions are widely developed over the Japanese islands, with some spotted high-Q regions. Relatively high Q is observed in most regions of the Korean Peninsula except a southeastern part where a Cretaceous volcanic-sediment basin exists. The high-Q values are close to the typical Q value in continental crust. Significantly low Q of 100 or less is observed in most oceanic regions including the East Sea (Sea of Japan), the Pacific Ocean, and the South China Sea. The high attenuation of Lg in oceanic regions suggests possible underestimation of magnitudes of oceanic events. Thus, it appears that proper correction of Lg amplitude is highly desired for accurate estimation of magnitudes of regional oceanic events.