  We present a weak-lensing analysis for the merging galaxy cluster, CIZA
J2242.8+5301, hosting double radio relics, using three-band Subaru/Suprime-Cam
imaging ($Br'z'$). Since the lifetime of dark matter halos colliding into
clusters is longer than that of X-ray emitting gas halos, weak-lensing analysis
is a powerful method to constrain a merger dynamics. Two-dimensional shear
fitting using a clean background catalog suggests that the cluster undergoes a
merger with a mass ratio of about 2:1. The main halo is located around the gas
core in the southern region, while no concentrated gas core is associated with
the northern sub halo. We find that the projected cluster mass distribution
resulting from an unequal-mass merger is in excellent agreement with the curved
shapes of the two radio relics and the overall X-ray morphology except for the
lack of the northern gas core. The lack of a prominent radio halo enables us to
constrain an upper limit of the fractional energy of magneto-hydrodynamics
turbulence of $(\delta B/B)^2<\mathcal{O}(10^{-6})$ at a resonant wavenumber,
by a balance between the acceleration time and the time after the core passage
or the cooling time, with an assumption of resonant acceleration by
second-order Fermi process.
