We analyzed long-term gravity data obtained from two LaCoste & Romberg gravimeters without electrostatic feedback systems at Shizuoka and Omaezaki in Tokai district, central Japan, and investigated the detectabilities of nontidal and tidal gravity changes at both stations. As for the nontidal gravity changes, it is difficult to detect meaningful signals in the order of 1 pgal/year because of large and irregular drifts of gravimeters On the other hand, δ-factors fluctuated to the amount of 10% at the most, when the data length was taken to be one month. These fluctuations were caused by the sensitivity changes of gravimeters. By employing the data length of twelve months, fluctuations were reduced to 3%, but we could not find out common features of δ-factor fluctuations between Shizuoka and Omaezaki during the periods of 1986-1988 and 1990-1992, when earthquakes larger than M=7 did not occur within the distance of 100 kmfrom Shizuoka and Omaezaki. Our observational result shows that δ-factor fluctuations more than 1% cannot be expected before and after earthquakes smaller than M=6.5 at the epicentral distance of 100 km.
