  We observed the atomic $1s$ and $2p$ states of $\pi^-$ bound to ${}^{121}{\rm
Sn}$ nuclei as distinct peak structures in the missing mass spectra of the
${}^{122}{\rm Sn}(d,{}^3{\rm He})$ nuclear reaction. A very intense deuteron
beam and a spectrometer with a large angular acceptance let us achieve
potential of discovery, which includes capability of determining the
angle-dependent cross sections with high statistics. The $2p$ state in a Sn
nucleus was observed for the first time. The binding energies and widths of the
pionic states are determined and found to be consistent with previous
experimental results of other Sn isotopes. The spectrum is measured at finite
reaction angles for the first time. The formation cross sections at the
reaction angles between 0 and $2^\circ$ are determined. The observed
reaction-angle dependence of each state is reproduced by theoretical
calculations. However, the quantitative comparison with our high-precision data
reveals a significant discrepancy between the measured and calculated formation
cross sections of the pionic $1s$ state.
