A Phase II/ III multicenter, open-label, single-group, designed to evaluate the efficacy and safety of study drug for the treatment of the MPS II.

Inclusion Criteria:

1. Capable of providing written consent by himself (not mandatory for those aged under 20 years at the time of informed consent process, or those who is impossible to obtain consent from the patient himself due to intellectual disabilities associated with MPS II.)
2. In the case of a patient who is under the age of 20 years or from whom it is not possible to obtain consent due to intellectual disabilities associated with MPS II, he may be included if written consent can be provided by legal representative (however, written consent should be obtained from the patient himself too, whenever possible)
3. Males with confirmed diagnosis of MPS II, based on deficient activity of iduronate-2-sulfatase (IDS) in leucocytes, plasma or fibroblasts and/or pathogenic mutations identified in the IDS gene, etc.
4. Naïve patients or patients who are receiving stable enzyme replacement therapy with Elaprase for more than 8 weeks before the observational period starts.

Exclusion Criteria:

1. Previous engrafted HSCT, excluding those who need enzyme replacement therapy even after HSCT.
2. Judged by the investigator or subinvestigator as being unable to undergo lumbar puncture, including those who have difficulties in taking position for lumbar puncture due to joint contracture or those who are likely experience difficulty breathing during the lumbar puncture process.
3. Judged by the investigator or subinvestigator to be ineligible to participate in the study due to a history of serious drug allergy or sensitivity.
4. Patients who have received other investigational product within 4 months before enrollment in the study.
5. Otherwise judged by the investigator or subinvestigator to be ineligible to participate in the study.