UPDATE 1-Airports of Thailand sees lower investment over 5 yrs

* Reviews five-year plan as construction costs drop

* To conclude investment plan in November

* Shares outperform in last three months (Adds quotes, details)

BANGKOK, Oct 26 (Reuters) - Airports of Thailand AOT.BK, the country's main airport operator, said on Monday investments over the next five years should cost less than a projected 80 billion baht ($2.39 billion) due to a decline in construction costs.

President Serirat Prasutanond told reporters the company would conclude its investment plans in November, mostly covering an expansion of Bangkok’s Suvarnabhumi airport, including a domestic terminal and a third runway.

“We will review our investment plans and want it lower. Construction costs have come down in line with weak demand and the poor economy. It should be a good opportunity for us,” Serirat said.

Majority state-owned AOT operates the country’s six main airports, Suvarnabhumi and Don Muang in Bangkok, Hat Yai, Chiang Mai, Chiang Rai and Phuket, which handle around 90 percent of air traffic in the country.

The company planned to spend 5.8 billion baht to expand capacity at Phuket airport to serve up to 12 million passengers a year versus 6 million now, Serirat said.

Another 6 billion baht would be spent to expand capacity at Suvarnabhumi airport to serve up to 65 million passengers a year, up from 45 million. The airport carries almost 40 million passengers a year, he said.

At 0841 GMT, AOT shares were unchanged at 40 baht, while the overall market was up 0.45 percent. In the past three months, the stock has risen 47 percent, outperforming the 18 percent rise in the market. ($1=33.41 Baht) (Reporting by Manunphattr Dhanananphorn; Writing by Khettiya Jittapong; Editing by Alan Raybould)