1. A search index generation apparatus, comprising: a computing device including a processor; a memory to store information of a first manipulation target processed in a first information manipulation performed by a user on a computer; and an information manipulation database, wherein the computing device includes: an information manipulation monitor unit to detect a second information manipulation performed by the user on the computer by monitoring the second information manipulation of the user on the computer; and a task detection unit to analyze information accumulated in the information manipulation database, to detect a first task of the user, to identify an information group processed in the first task, and to record the data relating to the first task in a task database, wherein the information manipulation database accumulates information of a second manipulation target processed in the second information manipulation detected by the information manipulation monitor unit, wherein the task detection unit judges whether the second manipulation target accumulated in the information manipulation database corresponds to the first manipulation target stored in the memory, maps the first manipulation target and the second manipulation target in a vector space to obtain a first manipulation target vector and a second manipulation target vector when the second manipulation target does not correspond to the first manipulation target, obtains similarity between the first manipulation target vector and the second manipulation target vector, adds the information of the second manipulation target to the memory when the similarity is higher than a threshold, and identifies the first and second manipulation targets stored in the memory as the information group processed in the first task, detects a second task of the user that handles the second manipulation target when the similarity is lower than the threshold, judges a third information manipulation performed by the user as an information manipulation to start a third task of the user when the third information manipulation relates to a start of access to a third manipulation target and there is no manipulation target being accessed to other than the third manipulation target, judges a fourth information manipulation performed by the user as an information manipulation to terminate the third task when the fourth information manipulation relates to a termination of access to the third manipulation target and there is no manipulation target being accessed to other than the third manipulation target, and detects the third task including the third information manipulation, the fourth information manipulation and a fifth information manipulation performed by the user between the third information manipulation and the fourth information manipulation.