  We examined data on about 20 years of observations of ER UMa available in
AAVSO, VSNET, AFOEV, NSVS, VSOLJ databases together with published light
curves. The obtained O-C diagram revealed a systematic change of the supercycle
(time interval between two successive superotbursts) within 43.6 and 59.2 d.
The time-scale of this change variation is from 300 to $\sim$ 1900 d. The
number of normal outbursts within the supercycles also varied from 4 to 6
although no strong correlation between this number and supercycle length was
found. We suggest that appearance of negative superhumps is responsible for the
observed variations of number of normal outbursts. Our results generally
confirms the expectations by the thermal-tidal instability theory.
