A Study to Evaluate the Safety, Efficacy, and PK of ONO-2017 in Japanese Patients With POS 2 to 17 Year Olds

Inclusion Criteria: * Japanese male or female patients aged 2 to under 18 years at the time of informed consent. * Patients diagnosed with epilepsy as having POS with uncontrolled seizures at least 6 months prior to informed consent, regardless of the presence or absence of secondarily generalized seizures. * Patients who have had POS at least once in 4 weeks before registration. Seizure information can be obtained from the participant's own retrospective patient epilepsy diary, etc. * Participants must have been treated with 1 to 3 ASMs at stable doses for at least 2 months before registration. Exclusion Criteria: * Patients with a history of status epilepticus requiring hospitalization within 3 months before registration. * Patients with a history of non-epileptic psychogenic seizures. * Patients with simple partial seizures without motor symptoms or idiopathic generalized epilepsy. * Patients diagnosed with Lennox-Gastaut syndrome. * Patients with a history of serious drug-induced hypersensitivity reaction (e.g., Stevens-Johnson syndrome, toxic epidermal necrolysis, DRESS, drug-induced hypersensitivity syndrome \[DIHS\]) or drug-induced rash requiring hospitalization.