Determining confidentiality of an office document shared by multiple organizations. Each block of a document data set is stored in association with confidentiality information indicating whether the block is confidential. The document data set is dividable into blocks each being a unit including properties evaluated as having a certain characteristic. A document data set targeted for the confidentiality determination is acquired, and it is determined whether a document data set, including a block similar to each block of the acquired document data set, is stored. If the document data set including the similar block is stored, it is determined whether the confidentiality information indicating that the block is confidential is assigned to the block of the acquired document data corresponding to the similar block. If the confidentiality information indicating that the block is confidential is assigned, the acquired document data set is determined as confidential.