  Light curve observations of a recently discovered interstellar object
1I/`Oumuamua suggest that this object has an extremely elongated shape with the
axis ratio 0.3 or smaller. Planetesimal collisions can produce irregular shapes
including elongated shapes. In this paper, we suggest that the extremely
elongated shape of 1I/`Oumuamua may be the result of such an impact. To find
detailed impact conditions to form the extremely elongated objects, we conduct
numerical simulations of planetesimal collisions using Smoothed Particle
Hydrodynamics method for elastic dynamics with self-gravity and interparticle
friction. Impacts into strengthless target planetesimals with radius 50 m are
conducted with various ratios of impactor mass to target mass q, friction
angles phi_d, impact velocities v_imp, and impact angles theta_imp. We find
that impacts with q \geq 0.5, phi_d \geq 40 degrees, v_imp \leq 40 degrees, and
theta_imp \leq 30 degrees produce remnants with the ratio of intermediate to
major axis length less than 0.3. This impact condition suggests that the parent
protoplanetary disk in the planetesimal collision stage was weakly turbulent
(alpha < 10^{-4} for the inner disk) and composed of planetesimals smaller than
~ 7 km to ensure small impact velocity.
