  We discuss excitation of string oscillation modes by an initial singularity
of inflation. The initial singularity of inflation is known to occur with a
finite Hubble parameter, which is generally lower than the string scale, and
hence it is not clear that stringy effects become significant around it. With
the help of Penrose limit, we find that infinitely heavy oscillation modes get
excited when a singularity is strong in the sense of Krolak's classification.
We demonstrate that the initial singularities of Starobinsky and hill top
inflation, assuming the slow roll inflation to the past infinity, are strong.
Hence stringy corrections are inevitable in the very early stage of these
inflation models. We also find that the initial singularity of the hill top
inflation could be weak for non-slow roll case.
