A data masking system comprising: a data conversion unit that converts a part including secret target information of all or part of first data including the secret target information, into second data for reading or viewing; a masking processing unit (106, 108, 110, 111) that performs masking processing on the second data, thereby generating masking data; a storage unit that stores the masking data; and an output unit that outputs the masking data stored in the storage unit characterized in that the data masking system further comprises a secret part selecting unit that selects a secret target region including the secret target information included in the second data, wherein the masking processing unit (106, 108, 110, 111) comprises: a first masking processing unit that performs selective masking processing on the secret target region selected by the secret part selecting unit to generate the masking data; a second masking processing unit that performs batch masking processing on the whole of the second data to generate a batch masking data; a temporary storage unit temporarily stores the batch masking data; and a masking canceling unit that cancels the batch masking processing of the batch masking data read out from the temporary storage unit to reconstruct the second data, wherein the first masking process unit performs the selective masking processing on the second data reconstructed by the masking canceling unit. a data conversion unit that converts a part including secret target information of all or part of first data including the secret target information, into second data for reading or viewing; a masking processing unit (106, 108, 110, 111) that performs masking processing on the second data, thereby generating masking data; a storage unit that stores the masking data; and an output unit that outputs the masking data stored in the storage unit characterized in that the data masking system further comprises a secret part selecting unit that selects a secret target region including the secret target information included in the second data, wherein the masking processing unit (106, 108, 110, 111) comprises: a first masking processing unit that performs selective masking processing on the secret target region selected by the secret part selecting unit to generate the masking data; a second masking processing unit that performs batch masking processing on the whole of the second data to generate a batch masking data; a temporary storage unit temporarily stores the batch masking data; and a masking canceling unit that cancels the batch masking processing of the batch masking data read out from the temporary storage unit to reconstruct the second data, a first masking processing unit that performs selective masking processing on the secret target region selected by the secret part selecting unit to generate the masking data; a second masking processing unit that performs batch masking processing on the whole of the second data to generate a batch masking data; a temporary storage unit temporarily stores the batch masking data; and a masking canceling unit that cancels the batch masking processing of the batch masking data read out from the temporary storage unit to reconstruct the second data, wherein the first masking process unit performs the selective masking processing on the second data reconstructed by the masking canceling unit.