I am a Canadian, I am going on a Working Holiday in Japan for 1 year, but I don't know should I buy a Travel Insurance in Canada?
Or is better to buy the insurance in Japan?
Just to answer for myself, I have already moved to Osaka, Japan for almost one year. Like Kent said, when I arrived to Osaka, I could enroll in the National Health Insurance Plan (国民健康保険).
The Working Holiday period given to me was 6 months (I could extends to 1 year after arrived for 3, 4 months... forgot how many months exactly).
But either way, I don't have any problem enrolled into the National Health Insurance Plan. The price for 6 months insurance is also really cheap, merely around 2000 yen ($20 CAD) because they will file you as no employment in Japan in the previous year and give you a big discount.
So yes, buying insurance in Japan is way better.
As a resident of Japan you are required to sign up to the National Health Insurance Plan, which you pay for monthly and covers 70% of your healthcare costs. As you said, the first year is very cheap.
But, as a foreigner you have more needs than a Japanese person, for example possible evacuation in a medical, natural, or political disaster, or getting into legal trouble. So you should also consider insurance in your home country that covers you for repatriation, legal fees, or anything else you may need in addition to the Japanese health insurance. Read the details carefully though - I discovered my one year travel insurance policy was invalid because I hadn't bought a return plane ticket!