The Prius c has been a hit for Toyota, and the company can’t make enough of them. Toyota is shipping just 6,000 units of the compact hybrid to North America per month, which is not enough to satisfy demand, but that number is set to grow by mid 2012. The Prius c went on sale domestically in December last year, and hits US showrooms this month.

Supply constraints and soaring domestic market demand is keeping supplies low for the rest of the world. In Japan, the small car known as Aqua has racked up a big backlog of 120,000 orders. The rising yen (against USD) is also affecting profit margins for US sales, so it makes sense for Toyota to focus on fulfilling local orders.

The Prius c is exclusively made in Iwate, northern Japan, at a plant run by Toyota affiliate Kanto Auto Works Ltd. Kanto president Tetsuo Hattori revealed that the company is currently working on rolling out more units of the small Prius.

Besides improving efficiency that will increase overall capacity, the plant will gradually shift production of the JDM Toyota Ractis to a sister plant in Higashifuji from May. The freed up capacity will be used to make more Aquas.

Kanto’s Iwate plant makes about 30,000 units of the Prius c units a month. The annual rate of 360k units is already up from the originally planned 240k units, and it accounts for 80% of total plant output. Besides the Prius c and Ractis, the facility also produces the Toyota ist (called Urban Cruiser in UK and Scion xD in USA), Belta (we’re very familiar with this, the Vios) and the Blade hatchback.

The Prius c was launched in Malaysia last month, click here to read our launch report.