A method for identifying a key for associating messages include: extracting attribute values for a first attribute, which are included in messages, and appearance times of the messages, from a message storage unit storing messages, each including one or plural attribute values for one or plural attributes, and appearance times of corresponding messages; calculating, for each of the extracted attribute values for the first attribute, a difference between an earliest appearance time and a latest appearance time among the appearance times of the messages including a corresponding attribute value; determining whether or not a ratio of differences that are shorter than a predetermined period is equal to or greater than a predetermined threshold; and upon determining that the ratio is not less than the predetermined threshold, storing data representing the first attribute is a candidate of a key for associating the messages stored in the message storage unit.