For the purpose of studying source mechanism of earthquakes and volcanic activities, we use interferometric technology applied for JERS-1 SAR data. Since 1994 we detected crustal deformations associated with major earthquakes in and around Japan: 1995 Hyogo-ken Nanbu earthquake, 1995 Northern Sakhalin (Neftegorsk) earthquake, persistent earthquake swarm in Izu Peninsula (Fujiwara et al., 1997), March 1997 Kagoshima-ken Hokuseibu earthquake (Tobita et al., 1997b) are among those. The most recent one is 1998 South of Iwate-san earthquake just occurred one month ago. At the same time we have been monitoring crustal deformations nationwide using a continuous GPS network system. Through experiences using both space technologies we learned that both are complimentary to each other in many respects and a reasonable combination of both is highly beneficial. In this presentation, we will discuss the advantages and some limitations of a reasonable combination of GPS and InSAR.