To provide a transformer device having a built-in tap changer capable of ensuring sealability even in special conditions in a cold district.

The transformer device includes a tap changer including a transformer tank 1 for housing the transformer in the inside, a tap changer 2 having a changing mechanism unit 6 fixed in the transformer tank and selecting a predetermined tap voltage, and an operation handle 4 for rotating and operating the tap changer by operating it from the outside of the transformer tank. The transformer further includes a tap changer comprising an operation coupling mechanism 5 formed between the operation handle and the tap changer, transmitting the rotation operation of the operation handle to the tap changing unit to rotate and operate the changing mechanism unit of the tap changer, a through-hole 1b formed on the transformer tank to allow the operation coupling mechanism to penetrate therethrough, and at least two or more cold-proof O-rings 3 formed in the through-hole to seal the inside and outside of the transformer tank.