IF YOU'RE LOOKING for an authentic recreation of Japan, Kotobukiya is not the place for you. It's a sushi bar and grocery store for those who already know what Japanese food is all about. Both are located in the Porter Exchange Mall in the old Sears building at 1815 Mass. Ave. in Porter Square and both provide inexpensive, delicious and undoubtedly genuine Japanese food.

The Kotobukiya sushi bar is a no-frills establishment. There is only room for about fifteen at the bar and you'll have to wait for a stool during dinner time. However, it's well worth the wait. Though the portions are on the small side, the sushi is fresh and all made right before your eyes by laconic, deft sushi chefs. The menu is in Japanese and English, but I got the feeling that Kotobukiya was designed for people who knew their Japanese food. While most Japanese restuarants provide elaborate descriptions of what kinds of fish they serve for the first-time sushi eater, there is only one bright placard explaining the different kind of sushi, though it is tucked behind the bar and difficult to see. Instead, you'll have to trust your instincts and bring a friend who knows the difference between sashimi and sushi. No Western-style utensils are provided, and if you don't know how to use chopsticks, you'll learn fast. But you can afford to experiment a little: A California roll is $2.50, and miso soup is only a dollar. Comparably priced sushi in the Square is premade and is already hours--if not a day--old.

Kotobukiya's grocery store is clearly for the connoisseur. The shelves are stocked with foods from Japan, but if you don't know exactly what you're looking for and don't speak Japanese, you'll be wandering a foreign country. Although little or no explanation is provided in English, if you want Japanese vegetables, noodles or even Japanese pancake mix, Kotobukiya will have it. You can get any variety of Japanese newspaper, magazine or manga (Japanese comics). The comperhensive array of food, Japanese-style cups and bowls and goods gave me the feeling that Kotobukiya served a large and loyal following. If you can only get your favorite toothbrush and hairbrush in Japan and nothing in CVS will do, Kotobukiya will have it. After your meal at the sushi bar, you may wish to emulate a sushi chef. Kotobukiya will provide you with pre-cut fish, and you can use your newfound knowledge and expertise to make Japanese delicacies of your own.