  Positronium is an ideal system for the research of the QED, especially for
the QED in bound state. The discrepancy of 3.9\sigma is found recently between
the measured HFS values and the QED prediction ($O(\alpha^3)$). It might be due
to the contribution of the unknown new physics or the systematic problems in
the previous all measurements. We propose new method to measure HFS precisely
and directly. A gyrotron, a novel sub-THz light source is used with a
high-finesse Fabry-P\'erot cavity to obtain enough radiation power at 203 GHz.
The present status of the optimization studies and current design of the
experiment are described.
