The business sentiment of South Korean firms worsened in May after improving for three straight months, pulled down by a decline in the advertising and real estate industries.

According to the Bank of Korea on Wednesday, the business survey index for all industries came to 73 this month, down one point from a month earlier.

A reading below 100 means pessimists outnumber optimists. 

The index posted 69 in January and rose for three consecutive months to 74 in April.

South Korean manufacturers' business sentiment increased one point to 76 this month, while the index for the non-manufacturing sector slipped three points to 71.

Local companies' outlook for all industries for next month was 73, a four point decline from the previous month.