  PSR J2021+4026 is a radio-quiet gamma-ray pulsar and the first pulsar that
shows state change of the gamma-ray emission and spin-down rate. The state
change of PSR J2021+4026 was first observed at 2011 October, at which the
pulsar changes the state from high gamma-ray flux/low spin-down rate state to
low gamma-ray flux/high spin-down rate st\ ate. In December 2014, PSR
J2021+4026 recovered the state before the 2011 state change over a timescale of
a few months. We report that the long term evolution of the gamma-ray flux and
timing behavior suggests that PSR J2021+4026 changed the state near 2018
February 1st and entered a new low gamma-ray flux/high spin-down rate state. At
the 2018 state change, the averaged flux dropped from $(1.29\pm 0.01)\times
10^{-6} {\rm cts~cm^{-2}s^{-1}}$ to $(1.12\pm 0.01)\times 10^{-6} {\rm
cts~cm^{-2}s^{-1\ }}$, which has the similar behavior to the case of 2011
event. The spin-down rate has increased by $\sim 3\%$ in the new state since
the 2018 state change. The shapes of pulse profile and spectrum in GeV bands
also changed at the 2018 event, and they are consistent with behavior at the
2011 state change. Our results probably suggest that PSR J2021+4026 is
switching between different states with a timescale of several years, like some
radio pulsars (e.g. PSR~B1828-11). PSR J2021+4026 will provide a unique
opportunity to study the mechanism of the state switching.
