A positron timing counter (TC) required 30-40 ps time resolution for ~50 MeV/c positron by the MEG II experiment has been developed. We employed the high segmented design with 512 scintillator plates ($120 \times 40 \times 5$ mm$^3$ and $120 \times 50 \times 5$ mm$^3$) attached 6-SiPM-array at the both ends. Pile up is reduced by the segmented design, and multi-counter measurement improves the overall timing resolution.
All the single counters were assembled and their resolutions were under 100 psec in pre-test with Sr source. The construction and installation was also completed in 2017. An engineering run was performed in the end of 2017 at the $\pi$E5 muon beam line in PSI, which supplies most intense muon beam ($7 \times 10^7$ stops/s on a target) in the world. We successfully operated the full system of TC in the MEG II environment and achieved the time resolution under 40 ps with more than 8 counter hits.
The overall resolution for the signal positrons is estimated to be 38.5 ps by weighting with the number of hits distribution of the signal positrons obtained by MC. The time resolution of TC is improved by a factor of 2 from the MEG experiment.