To examine inshore radioactive contamination caused by nuclear power plants, nuclear testing, hospital and laboratory, both gross ${eta}$-radioactivity and ${upsilon}$-spectrometry were measured. The measurements were taken with the Abalone and Top shell, which had been collected from the four different sites at the coast in Jeju-do (Seongsan, Deajeong, Wimi, Dodu). The activity ranges of concentration were respectively 0.11-0.29 Bq/kg-wet for $^{90}Sr$, 0.01-0.04 Bq/kg-wet for $^{137}Cs$, 36 - 53 Bq/kg-wet for $^{40}K$ in shellfish on the Jeju island. The radioactivities of $^{90}Sr$, $^{137}Cs$ and $^{40}K$ were similar to those in sampling location of shellfish in the coastal on the Jeju island. The radioactivity of $^{90}Sr$ for the Abalone was lower level than that for Top shell. The radio- activity of $^{137}Cs$ for the Abalone was similar to that for the Top shell. But the radioactivity of $^{40}K$ for the Abalone was higher level than that for the Top shell. As a result, the gross ${eta}$-radioactivity observed was similar to that in nature. Among radionuclides, only $^{90}Sr$, $^{137}Cs$ and $^{40}K$ were detected in a very small amount, and each was also close to the natural levels.