Japan's coronavirus guidelines encourage people to wear masks but the medical body warned parents not to put them on infants because it makes it difficult to notice changes in face color, expression and breathing, it said in a leaflet.
"It is possible that masks make it difficult for infants to breathe and increase the risks of heat stroke," reads the leaflet.
Infants have narrower airways and masks can make it more difficult to breathe, increasing the burden on their lungs, it continues.
There is also an increased risk of suffocation, particularly if small children vomit behind a mask.
Infants are relatively low risk for coronavirus infections and the association concludes that masks are not necessary for infants under two years old.
On Monday Japan's Prime Minister Shinzo Abe lifted the country's nationwide state of emergency. It had been in place almost a month but authorities lifted it a week earlier than originally planned.
However, Abe expanded a travel ban to 111 countries effective Wednesday, now including the United States, India, and South Africa.
The ban list expanded by 11 countries this week and forbids foreign nationals who stayed in those countries from entering Japan.
Japanese citizens are still