A morphological study of coronal weak activity visible in 17 GHz microwave emission is presented from the Nobeyama Radioheliograph observations. Including four preflare events we analyze six events located at or near the solar limb, all of which clearly show ``microwave blobs'' above arcade-like structures of coronal magnetic loops. The brightness temperatures of the blobs range from 1 X 10^4 K to 3 X 10^4 K, while that of quiet regions on the solar disk is about 1 X 10^4 K. From comparisons between the radio images and the Yohkoh SXT images, we find that the origins of the microwave blobs can be divided into two groups; i) remnants of flares and ii) evaporated plasma from the lower atmosphere (presumably the chromosphere). On the basis of the morphological evolution of the microwave-emitting structures, a possible energy release process in arcade-like coronal loops is briefly discussed.