Problem To provide a method for easily and stably storing P1 P4 bis(5 uridyl)tetraphosphate crystals for a long period of time. Solution A method for storing P1 P4 bis(5 uridyl)tetraphosphate crystals or a pharmaceutically acceptable salt thereof in a package which is characterized by storing under one condition selected from among the storage conditions (1) (3) described below. (1) The storage temperature is 0C or higher but lower than 25C. (2) The storage temperature is 25C or higher but lower than 40C and the crystal pH is 4.5 8.0. (3) The storage temperature is 40C or higher but lower than 60C and the crystal pH is 5.0 6.4.