BOK officials attributed the fall to the deadly sinking of the ferry Sewol that has hurt consumer sentiment.
The figure is the lowest since September last year when the index stood at 102.
A reading above 100 on the index means optimists outnumber pessimists.
The May index is based on a survey of 2,200 households nationwide, which was conducted between May 13 and 20.
The ferry sank off the country's southwest coast on April 16, leaving more than 300 people, mostly schoolchildren, dead or missing.
reporter
Park Sae-jin
swatchsjp@ajunews.com
