Postseismic deformation following the Aichiken-Tobu earthquake (Mw=5.7), which occurred on March 16, 1997 in central Japan, was detected by strainmeter networks. We show that this deformation can be consistently explained by slow slip on the fault, total amount of which is comparable to the high-speed (coseismic) slip. The time constant of the slow after-slip fitted to a logarithmic function is estimated to be roughly 0.25 hour. Comparison of this feature with that of Sanriku-Haruka-Oki earthquake (Mw=7.6), in which significant slow slip was detected by a GPS network, suggests a direct proportionality of the time constant and seismic moment of coseismic slip. We could detect very small signals (strain between 10-7 and 10-8) associated with this moderate-size earthquake due to the high sensitivity of strainmeters. Strainmeters are especially useful in detecting afterslip from smaller earthquakes. ¿ 1999 American Geophysical Union