I would like to know about the overweight baggage.

The baggage that weighs 20kg to 32kg is considered as heavy baggage. Extra charge applies to heavy baggage in addition to checked baggage charges.
Heavy baggage charges are the same for all fare types. The charges must be paid for each sector of the whole journey.

Please see below for the fees and charges.
「Domestic Flights: Checked Baggage Charges」
「International Flights: Checked Baggage Fees」

• Website
  Please log in to the "Manage Booking" page below and complete the procedure no later than 1 hour prior to the departure time stated in the itinerary.
  「Manage booking」（Log in page）
• Contact Center
  You need to apply 1 hour prior to the scheduled departure time.
  「Apply by a Live chat」
• Airport Counter
  You can apply at the airport counter on the day of departure.