This paper discusses a problem of a diffraction of plane transient waves in an infinite elastic medium with two infinitely long parallel cylindrical cavities to an impact load. In the analysis, it is assumed that an axially symmetric impact load acts at the interface of one of the cavities which is small enough compared with the others. Expressions of stresses around the large cavity are obtained by the Fourier transform method for an exponentially decaying impact load. Variations of a tangential stress around the cavity with time are shown in a numerical example.