I'm a former Flight Attendant and I fly a lot internationally with my three children.
The truth is that it is easier and more comfortable to have the baby in his or her own seat. Babies under 2 are allowed for commercial reasons and because air travel is so safe, there's little chance of anything going wrong. Parent save money, the airlines sell more tickets and everyone is happy...as long as nothing goes wrong!
But there is a catch. A lot of airlines have funky rules regarding car seats. I've had mine taken away, forced to check them, etc. because they decided that my seat was of the "wrong" nationality. One company even outright banned all car seats from the cabin.
In the U.S., if the parent buys a ticket for the baby and has a FAA approved (most sold there are) car seat, they have the right to use it. Car seat rules elsewhere in the world are a lot less precise.
So before you buy a ticket for him or her, find out what their rules are regarding car seats on board. Find out specifically if the one you have will be accepted.
It's really a bad idea to check car seats in as luggage. They could too easily be lost and/or damaged in the hold. If you can't use your seat, try to take it to the gate and have it "gate-checked". It's a bit better than checking it in as luggage.
Some airlines do offer wall-mounted bassinets. These are convenient if you don't have a seat for your baby but are NO replacement for a car seat as far as safety is concerned.
First try your airlines' website to see if they have that information. If not, try emailing the company. Email is better than phoning as you can print up the response and show it to anyone who tells you anything different.
I have lots of tips on flying with babies. Nine years ago I wrote an article on this subject. It's based on both my personal and professional experience of flying with children. I go into more in-depth detail on feeding, packing, getting the car seat through the airport, etc.
It's totally non-commercial and other parents from around the world have contributed;
Have a good time in Japan and good luck being able to use your car seat on board!
Former Flight Attendant, 13 years, 2 companies, almost all long-haul international
3 children, now ages 9, 7 & 5, flying since each was 4 months old, between Europe and California about twice a year plus other shorter flights in between, regularily scheduled, low cost, charter, etc.