The double differential cross sections for $^{nat}$In($^{28}$Si, $pi ^{pm}$) reactions are measured at 400, 600, and 800 MeV/nucleon. Both $pi^+$ and $pi^-$ are found to be emitted isotropically from a single moving source. The $pi^- / pi^+$ yield ra
tio is determined as a function of the charged pion energy between 25 and 100 MeV. The experimental results significantly differ from the prediction of the standard transport model calculation using the code PHITS. This discrepancy suggests that more theoretical works are required to deduce firm information on the nuclear symmetry energy from the $pi^- / pi^+$ yield ratio.
