The liquid xenon (LXe) gamma-ray detector in the MEG II experiment measures the energy, position and timing of the gamma-ray from μ⁺ → e⁺γ, and it is the key to the unprecedented sensitivity of the experiment. All the photo sensors of 4092 VUV MPPCs and 668 PMTs were read out for the first time and a physics data collection started in 2021. The detector response was monitored all through the beam time, and the LXe detector operated stably. The timing and energy resolution were measured using the gamma-rays from the π⁰ decays after charge exchange reaction of charged pions in a liquid hydrogen target. The detector has been successfully commissioned and is ready for the long physics run.