The Japan Meteorological Agency announced on Monday that the El Nino phenomenon, which is believed to affect the world's climate, seems to have ended.
El Nino refers to a situation when the ocean surface becomes warmer than usual near the equator off the South American country of Peru.
The agency says the phenomenon began early last year, but the latest data shows temperatures are falling.
Experts say that when El Nino ends, high-pressure systems over the Pacific expand, causing a warm and damp air mass to flow over Japan.
The agency forecasts that average temperatures in the three months until August will be higher than usual throughout Japan.
Senior agency official Tanaka Shotaro says a cold air mass has kept temperatures relatively low recently, but hotter weather is forecast in coming months.
Tanaka urges people to take precautions to avoid developing heatstroke.