Welcome @parktaekyu and @snudge, There are quite a few members with PMR who have used Methotrexate to help taper off of prednisone. Here's a discussion that might be helpful:
-- Methotrexate to wean off long term Prednisone for PMR
https://connect.mayoclinic.org/discussion/methotrexate-to-wean-off-long-term-prednisone-for-pmr/
My PMR is currently in remission but I've had two episodes, the first one took 3 and half years to taper off and the PMR came back 6 years after remission. It took me 1 and half years to taper off the second time. What helped me during both occurrences was keeping a daily journal of my symptoms when I got up in the morning and my prednisone dosage for that day. That way I could track my level of pain and use it when tapering to the next lower dose. My tolerable pain level was 0 to 2 on a scale of 0 to 10. If it was time to taper to the next lower dose and my pain was above 2, I would wait a few days to see if it improved before going to the next lower dose. If it was really a jump in pain, I would increase my dose by half of what I previously tapered down. Each of us are different and it might be a good to have a discussion with your doctor or rheumatologist.

Do you keep a daily journal of your pain and dosage?