Among 13 BL Lacs observed with Ginga, 1H 0323+022, Mkn 421 and PKS 2155-304 exhibited significant variablity during each (typically one day) observation. On the flux-hardness plane, the data points obtained from each source draw a sort of clockwise hysteresis motion. It means that the spectrum hardens before the source gets brighter, while the spectrum softens before the source becomes fainter. Such a soft-lag behavior, first pointed out for PKS2155-304 by Sembay et al. These properties were also confirmed with the discrete cross correlation function technique.