Provided are a co-occurrence word data generating unit configured to generate co-occurrence word data including a co-occurrence word that is a vocabulary used along with a predetermined keyword in posted data in all periods and an appearance frequency of the co-occurrence word, from among pieces of posted data posed in a plurality of different periods; and a related data generating unit configured to generate related data including the co-occurrence word as a usual related word when a temporal variation in the appearance frequency of the co-occurrence word is smaller than a first threshold value and the appearance frequency is higher than a second threshold value.
Please help me write a proper abstract based on the patent claims. CLAIM: 1. A related data generating apparatus, comprising: a co-occurrence word data generating unit configured to generate co-occurrence word data including a co-occurrence word that is a vocabulary used along with a predetermined keyword in posted data in all periods and an appearance frequency of the co-occurrence word, from among pieces of posted data posted in a plurality of different periods; and a related data generating unit configured to generate related data including the co-occurrence word as a usual related word when a temporal variation in the appearance frequency of the co-occurrence word is smaller than a first threshold value and the appearance frequency is higher than a second threshold value. 2. The related data generating apparatus according to claim 1, wherein the related data generating unit stores the co-occurrence word as a usual related word in the related data, in a case in which the appearance frequency of the co-occurrence word is lower than a third threshold value during a period prior to a recent period, in a case in which the appearance frequency of the co-occurrence word is higher than a fourth threshold value higher than the third threshold value during the recent period, in at least one of a case in which a temporal variation of the appearance frequency of the co-occurrence word is larger than the first threshold value and a case in which the appearance frequency of the co-occurrence word is smaller than the second threshold value. 3. The related data generating apparatus according to claim 1, wherein the related data generating unit calculates a score having a larger value as the appearance frequency of the co-occurrence word increases, and stores the score in the related data as a score of the usual related word corresponding to the co-occurrence word. 4. The related data generating apparatus according to claim 3, wherein the related data generating unit stores the co-occurrence word of which a score is greater than the score of the usual related word stored in the related data in the related data instead of the usual related word. 5. The related data generating apparatus according to claim 3, wherein the related data generating unit again calculate a score of the usual related word based on an appearance frequency of the usual related word in the period when the usual related word identical to the co-occurrence word is registered in the related data. 6. The related data generating apparatus according to claim 1, wherein the co-occurrence word data generating unit generates co-occurrence word data including a co-occurrence word which is a vocabulary used along with at least two different usual related words and an appearance frequency of the co-occurrence word in the posted data posted during a predetermined period, and the related data generating unit stores the co-occurrence word in the related data as a related word similar name when a similarity between the co-occurrence word and the predetermined keyword is larger than a fifth threshold value. 7. The related data generating apparatus according to claim 6, wherein the related data generating unit calculates a score having a larger value as a similarity between the co-occurrence word and the predetermined keyword increases and stores the score in the related data as a score of the usual related word similar name corresponding to the co-occurrence word. 8. The related data generating apparatus according to claim 6, further comprising a display information generating unit configured to generate display information for displaying the usual related word along with a target indicated by the predetermined keyword. 9. The related data generating apparatus according to claim 6, further comprising a trend level data generating unit configured to: calculate a degree at which the predetermined keyword or the related word similar name to which the predetermined keyword is mapped is identical to a vocabulary included in the posted data posted during the predetermined period as a first coincidence degree, calculate a degree at which the related word similar name to which the predetermined keyword is mapped is identical to a vocabulary included in the posted data posted during the predetermined period as a second coincidence degree, calculate a value obtained by summing up the first coincidence degree and the second coincidence degree calculated for all pieces of posted data within the predetermined period, as a trend level of a target indicated by the keyword, and generate a trend level data in which the trend level is stored to be mapped to the predetermined keyword. 10. The related data generating apparatus according to claim 9, wherein the trend level data generating unit is configured to: set a predetermined value as the first coincidence degree when the predetermined keyword is identical to a vocabulary included in the posted data, determine whether the related word similar name mapped to the keyword is identical to the vocabulary included in the posted data when the predetermined keyword is not identical to the vocabulary included in the posted data, and set the first coincidence degree, which is a value smaller than the predetermined value, by using a score of the related word similar name identical to the vocabulary when the related word similar name is identical to the vocabulary included in the posted data. 11. The related data generating apparatus according to claim 9, wherein the trend level data generating unit sets the second coincidence degree by using different values according to a score set in the usual related word when the usual related word is identical to a vocabulary included in the posted data. 12. The related data generating apparatus according to claim 9, wherein the display information generating unit generates display information indicating the trend level. 13. A related data generating method executed by a related data generating apparatus, comprising: generating co-occurrence word data including a co-occurrence word that is a vocabulary used along with a predetermined keyword in posted data during all periods and an appearance frequency of the co-occurrence word, from among pieces of posted data posed in a plurality of different periods; and generating related data including the co-occurrence word as a usual related word when a temporal variation in the appearance frequency of the co-occurrence word is smaller than a first threshold value and the appearance frequency is higher than a second threshold value. 14. A program for causing a computer to function as a related data generating apparatus that generates related data, the computer functioning as: a co-occurrence word data generating unit configured to generate co-occurrence word data including a co-occurrence word that is a vocabulary used along with a predetermined keyword in posted data during all periods and an appearance frequency of the co-occurrence word, from among pieces of posted data posed in a plurality of different periods; and a related data generating unit configured to generate related data including the co-occurrence word as a usual related word when a temporal variation in the appearance frequency of the co-occurrence word is smaller than a first threshold value and the appearance frequency is higher than a second threshold value.