  We report on the spectroscopy of the $5s^2 {}^1S_0 (F=9/2) \to 5s5p {}^3P_0
(F=9/2)$ clock transition of ${}^{87}{\rm Sr}$ atoms (natural linewidth of 1
mHz) trapped in a one-dimensional optical lattice. Recoilless transitions with
a linewidth of 0.7 kHz as well as the vibrational structure of the lattice
potential were observed. By investigating the wavelength dependence of the
carrier linewidth, we determined the magic wavelength, where the light shift in
the clock transition vanishes, to be $813.5\pm0.9$ nm.
