Today's Lesson Vol.019

Last week Taisei celebrated the 116th anniversary of the opening of the school.
Normally the ceremomny is held on May 30th, but this year it was held a little earlier. There was a short ceremony and after that the students enjoyed an interesting musical about Tohoku in the days after 2011 Tohoku Earthquake and Tsunami.

Which word completes this sentence?
Taisei was founded ______ 100 years ago.