Recently I checked the internet and found that piano tuning was very important. According to the article, the life of the piano starts the countdown from that year when we abandoned the tuning maintenance.That is, piano tuning is not just adjusting the sound.
So I asked Mr. Itasaki, the tuner from Sankyo Music instrument company to tune my mother's grand piano which had skipped the tuning last year. After finishing the tuning, he gave me a advice that the tuning maintenance might be better at the pace of every two years, if the piano is not played quite often. Because my mother only comes back home three times a year.
But this tuning maintenance has become a good chance for me to use the grand piano for my piano lesson.