A new teaching guideline released Monday calls for high school English classes to be taught primarily in English from the academic year beginning in April 2013.
The drive is aimed at enhancing students' English communication skills, according to the Education, Culture, Sports, Science and Technology Ministry.
Complicated explanations such as those about grammar may still be provided in Japanese, a ministry official said.
However, there is some concern among English teachers about the draft guideline. It is scheduled to be finalized for announcement in February or March.
"This could embarrass particularly older teachers who know grammar well but are not very good at conversation," one teacher said.
The number of English words to be taught during the three years at high school will be increased under the guideline to 1,800 from the current 1,300.
The number of English words to be taught at junior high school will be increased from the academic year beginning in April 2012 to about 3,000, 800 more than now.
Combined, the words would provide students with a vocabulary needed to communicate with people abroad, ministry officials said.
The level of the vocabulary would match that taught in China and South Korea, they added.