  The majority of massive stars are formed in binary systems. It is hence
reasonable to expect that most core-collapse supernovae (CCSNe) take place in
binaries and the existence of a companion star may leave some imprints in
observed features. Having this in mind, we have conducted two-dimensional
hydrodynamical simulations of the collisions of CCSNe ejecta with the companion
star in an almost-equal-mass ($\sim 10M_\odot$) binary to find out possible
consequences of such events. In particular we pay attention to the amount of
mass removed and its dependence on the binary separation. In contrast to the
previous surmise, we find that the companion mass is stripped not by momentum
transfer but by shock heating. Up to $25\%$ of the original mass can be removed
for the closest separations and the removed mass decreases as $M_{ub} \propto
a^{-4.3}$ with the binary separation $a$. By performing some experimental
computations with artificially-modified densities of incident ejecta, we show
that if the velocity of ejecta is fixed, the density of incident ejecta is the
single important parameter that actually determines the removed mass as $M_{ub}
\propto \rho_{ej} ^{1.4} $. On the other hand, another set of simulations with
modified velocities of incident ejecta demonstrate that the strength of the
forward shock, which heats up the stellar material and causes the mass loss of
the companion star, is actually the key parameter for the removed mass.
