Sales at convenience stores in Japan fell 1 percent on a same-store basis in December from a year earlier for the 18th consecutive monthly decline, the Japan Franchise Association said Monday.

Unstable weather was to blame for the decline in sales, the association said, claiming this discouraged consumers from buying.

But on an all-store basis, including newly opened stores, sales increased 2.3 percent to 602.65 billion yen, it said.

The association did not disclose the value of sales on a same-store basis.

The number of customers fell 0.9 percent on a same-store basis but increased 2.5 percent on an all-store basis.

Average per-customer expenditures slipped 0.6 percent on a same-store basis and 1.1 percent on an all-store basis, it said.

It said Christmas cakes, yearend gift products and New Year's greeting cards sold well.

The sales data cover a total of 36,932 outlets run by 13 major convenience store operators.