In this paper, the declarations (player's oaths) by senior high school baseball players at Koshien stadium every year are analyzed from the viewpoint of the modification of the texts. In result, 26 texts from 1968 to 2010 are divided into three groups according to the time. First group (named "traditional type") consists of the oaths from 1968 to 1983, that is traditional and stereotyped version made up of almost the same vocabularies and expressions. Second group ("innovation type Ⅰ") is from 1984 to 2000, that has variety and individualities. Third group("innovation type Ⅱ") is from 2001 ahead, that has the same character as the former but is different in that has entertainer's viewpoint in the texts. In conclusion, the reason why this kind of modification has occurred is that "liberation from restraint" emerged among Japanese young people after 1980s, and furthermore, "players as entertainers" consciousness has also emerged after 2000s.