The ministry decided to close the airport temporarily on Sunday because of volcanic ashes from Mount Soputan eruption.
North Sulawesi authorities on Sunday put Mt Suputan on the second highest alert status after it spewed ashes and hot clouds earlier in the day and then closed the airport.
But the Transportation Ministry`s Public Communication Center spokesman Bambang S Ervan said here on Monday the airport has been reopened.
"Sam Ratulangi airport has been reopened since 6.30 Central Indonesia Standard Time (WITA) Monday morning," Bambang said.
He said the reopening of the airport was announced in Notice to Airman No.A0876/11 issued by the Air Transportation Directorate General.
But Bambang added that airmen were also instructed to remain alert because volcanic ashes were still threatening air routes W15, W18, W51, W35, W36, W37, W32, W46, W55, W61, W67, A211, A461, B583, G576, G578 and R342.
Mt Soputan is one of the active volcanoes in North Sulawesi province. Almost every year, the 1,784-meter high volcano erupts and spews hot clouds.
According to data from the volcano`s monitoring post at Maliku village, Mt Soputan erupted at around 06.00 a.m. on Sunday morning, sending ashes up to 5,000 meters into the sky.
Therefore, the local authorities had ordered people living on the volcano`s slopes to keep a close watch on the eruption and stay at least 6 km from the crater.
At present local residents were still staying at their respective homes but local authorities were in a stanby status so as to ready for immediate action to evacuate them when necessary.