The father of a Japanese boy who received a heart transplant in the United States is asking for more donations as their family lacks funds, partly due to depreciation of Japan's currency.

Nakazawa Yuto, who is 11, developed restrictive cardiomyopathy -- a disease said to affect one in 500,000 people.

His family raised 237 million yen, or about 1.75 million dollars, in donations from across Japan. He traveled to the US in February and got the transplant in October.

Yuto has since been in good condition and is expected to return to Japan soon. But his father, Nakazawa Chiharu, says the family needs more money.

The father held a news conference at the Nagano prefectural office on Wednesday. The family lives in the prefecture. He said they need about 2.66 million dollars in new donations. He said Yuto also had surgery to use an auxiliary artificial heart while waiting for the transplant, and was in an intensive care unit. He cited the impact of the weak yen as well.

The father said the website of a group supporting the boy shows information on a bank account for donations, and that he is asking shops and other facilities in and outside the prefecture to install collection boxes.

Nakazawa said he is sorry to ask for more donations, but that he hopes people can help his son once again.