1. A non-transitory computer-readable recording medium having stored therein a program for causing a computer to execute a process for putting trouble handling cases occurring in the past in an information system into knowledge data, and for recommending a handling method using a trouble handling knowledge obtained by putting the trouble handling cases into the knowledge data, and a symptom of a trouble when the trouble occurs, the process comprising: obtaining candidates of a handling method for a trouble requested to be handled by searching the trouble handling knowledge; recording to a storing unit a history of handling methods executed for each symptom and a history of candidates of the handling method at the time of the execution as handling history information; assigning priorities to the candidates of the handling method obtained by the obtaining step in the handling history information recorded in the storing unit; returning to a handling request source the handling method for the trouble requested to be handled after assigning a priority to the handling method using priority assignment information obtained by the assigning; and integrating, into one, handling history information having a plurality of common portions in the handling history information recorded in the handling history management information, wherein: the assigning step assigns a higher priority to a handling method candidate effective also as to other symptoms being handled, as an appearance frequency of the candidates of the handling method in handling history information of the other symptoms being handled becomes higher, the appearance frequency recognized by referring to handling history information of the other symptoms being handled, and the integrating step refers to pieces of handling history information within the handling history management information, compares the pieces of handling history information to determine whether a ratio of common portions to an entire history of handling methods is equal to or larger than a certain value, and determines that there are a plurality of common portions when the ratio of common portions is equal to or larger than the certain value.
3. The non-transitory computer-readable medium according to claim 1 , wherein the assigning step assigns a higher priority to a more recently executed handling method candidate with reference to the handling history information.