The post and telecommunications group remained unchanged while other groups saw wild fluctuations, the report says.
Housing, electricity, water, fuel, and construction materials declined by 0.24-1.03 percent, while tobacco, textiles and garment, hats, shoes, and home appliances climbed up.
The reasons cited by economic experts included an increase in petrol and oil prices in late April and a rise in minimum wage level as of May 1. The long holiday in late April and early May also contributed to the rising CPI of goods and services related to culture, entertainment, and tourism.
Another reason was the falling prices of food and vegetables as well as restaurant services due to a stable supply from the markets.
The May index for gold prices decreased by 1.76 percent from April but rose 11.82 percent against the same month of last year.