Inclusion Criteria:

[Cohort of pediatric malignant solid tumor Tolerability evaluation part]

1. Patients aged ≥ 12 to < 18 years at the time of signing the informed consent form (ICF)

2. Patients with radically/curatively unresectable advanced or metastatic solid tumor who are refractory to or intolerant to standard treatment or for which no standard treatment is available.

3. Patients with ECOG PS of 0 to 1

   [Cohort of pediatric malignant solid tumor Expansion part]

4. Patients aged ≥ 12 to < 18 years at the time of signing the informed consent form (ICF)

5. Patients with unresectable tumors for whom nivolumab intravenous monotherapy is indicated in adults according to the package insert

6. Patients who have at least 1 measurable lesion per RECIST guideline

7. Patients with ECOG PS of 0 to 2

   [Cohort of Epithelioid Sarcoma]

8. Patients aged ≥ 12 years of age at the time of signing the informed consent form (ICF). However, patients aged ≥ 12 to < 18 years may be enrolled after tolerability is confirmed in the tolerability evaluation part in the cohort of pediatric malignant solid tumor.

9. Patients who are refractory to or ineligible to at least 1 regimen of chemotherapy including doxorubicin for unresectable advanced or recurrent epithelioid sarcoma

10. Patients who have at least 1 measurable lesion per RECIST guideline

11. Patients with ECOG PS of 0 to 2

    [Common]

12. Life expectancy ≥ 3 months at the time of enrollment

Exclusion Criteria:

1. Patients with current or previous severe hypersensitivity reactions to antibody products
2. Patients with primary central nervous system tumor
3. Patients with brain or meningeal metastases. However, patients who are asymptomatic and do not require treatment can be enrolled.
4. Patients with multiple cancers
5. Patients who have previously received anti-PD-1 antibody, anti-PD-L1 antibody, anti-PD-L2 antibody, anti-CD137 antibody, anti-CTLA-4 antibody, or other therapeutic antibodies or pharmacotherapies for regulation of T cells