According to a similar coin put up for auction in Yahoo Auction (Japan), it's a silver piece coin from the Tenpou Period (1830-1843). It is worth a little less than 3,000 yen. I read a comment that it shouldn't be washed or polished.
一分銀（いちぶぎん）("ichibu" silver coin)
定 （さだめる・てい）(kanji used to show that the coin is certified by the authorities)
銀堅 Probably read ぎんけん suggesting "hard silver"
常是（つねこれ）(reference probably to 大黒常是, a government official in charge of minting silver coins)