  Recent VLBI monitoring has found transverse motions of the M87 jet. However,
due to the limited cadence of previous observations, details of the transverse
motion have not been fully revealed yet. We have regularly monitored the M87
jet at KVN and VERA Array (KaVA) 22 GHz from December 2013 to June 2016. The
average time interval of the observation is ~ 0.1 year, which is suitable for
tracking short-term structural changes. From these observations, the M87 jet is
well represented by double ridge lines in the region 2 - 12 mas from the core.
We found that the ridge lines exhibit transverse oscillations in all observed
regions with an average period of $0.94\pm0.12$ years. When the sinusoidal fit
is performed, we found that the amplitude of this oscillation is an order of
$\sim0.1$ mas, and the oscillations in the northern and southern limbs are
almost in phase. Considering the amplitude, it does not originate from Earth's
parallax. We propose possible scenarios of the transverse oscillation, such as
the propagation of jet instabilities or magneto-hydrodynamic (MHD) waves or
perturbed mass injection around magnetically dominated accretion flows.
