Disclosed is a glass composition composed of an oxide glass wherein the ratios of constitutional elements other than oxygen (O) expressed in atomic % are as follows: boron (B) is not less than 56% and not more than 72%; silicon (Si) is not less than 0% and not more than 15%; Zinc (Zn) is not less than 0% and not more than 18%; potassium (K) is not less than 8% and not more than 20%; and the total of K, sodium (Na) and lithium (Li) is not less than 12% and not more than 20%. This glass composition may further contain at least one of magnesium (Mg), calcium (Ca), strontium (Sr) and barium (Ba) in an amount of more than 0% and not more than 5%, and molybdenum (Mo) and/or tungsten (W) in an amount of more than 0% and not more than 3%.