We evaluate how large interplate fault slip around Japan can be detected by the GNSS Earth observation network system (GEONET), based on the dislocation theory. The detectability is calculated in term of two characteristic parameters; threshold and number of station. Evaluation of detectability of interplate fault slip has indicated that GEONET have the monitoring capability at least Mw6.5 or more on the Pacific plate and Mw6.0 or more on the Philippine Sea plate, when the threshold and number of station are 7 mm in horizontal, 14 mm in vertical and 3 sites or more, respectively. It should be noted that the detectability depends on the assumed parameters, such as threshold, number of station and fault parameters. The detectability gets worse when the threshold or number of station is larger. The detectability greatly depends on the threshold than the number of station. The detectability also depends on the assumed fault parameters, and its difference causes magnitude gain or loss up to Mw0.5 at maximum. We consider that these results are a first step for understanding the detectability of the interplate fault slip on the Pacific plate and the Philippine Sea plate based on GEONET.