To investigate the efficacy and safety of ONO-2017 in combination with antiepileptics in Japanese epileptic patients with generalized tonic-clonic seizures.

Inclusion Criteria:

1. Gender and age: Japanese patients, regardless of gender, aged 12 years or older at the time of informed consent.
2. Subject has a clinical diagnosis of Primary Generalized Tonic-Clonic (PGTC) seizures in the setting of idiopathic generalized epilepsy.
3. Subject experiences at least 5 Primary Generalized Tonic-Clonic (PGTC) seizures in 12 weeks.
4. Subject is currently receiving 1 to a maximum of 3 concomitant Antiepileptic Drugs(AEDs) with fixed dosing regimens

Exclusion Criteria:

1. Subject has a history of status epilepticus that required hospitalization within 15 months prior to enrollment.
2. Subject has seizure clusters where individual seizures cannot be counted orclassified.
3. History of non-epileptic or psychogenic seizures.
4. Subject has a concomitant diagnosis of Partial Onset Seizure(POS).
5. Subject has a history of any serious drug-induced hypersensitivity reaction (including but not limited to Stevens Johnson syndrome, toxic epidermal necrolysis, drug reaction with eosinophilia and systemic symptoms(DRESS), Drug-induced hypersensitivity syndrome(DIHS)) or any drug-related rash requiring hospitalization.