Yusaku Maezawa wrote "YousuckMZ" on November 27.
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Mr Maezawa said the trip was a dream come true.
A billionaire said that people can have hopes and dreams if they see that.
He has promised to perform 100 tasks while on the International Space Station.
It can be expected, such as introducing his fellow astronauts and showing viewers what life is like on the International Space Station, or it can be more playful, such as playing golf, blowing bubbles, and throwing a paper aeroplane.
Mr Maezawa's voyage, which is reportedly costing him $88m, is the third trip by a billionaire to space this year, after Jeff Bezos' short journey to space and Richard Branson's trip to the edge of space.
The Japanese businessman has an eclectic background. The former drummer of a punk rock band founded Start Today in 1998 to sell rare CDs and records.
He became a billionaire when he was in his 30s, after he pivoted to fashion with Zozotown.
He is ranked as Japan's 30th richest man by Forbes magazine with an estimated personal wealth of $1.9 billion.
Mr Maezawa has a passion for space travel.
He was the first private passenger to be flown around the Moon by the company owned by Musk.
The flight is called dearMoon. In March, Mr Maezawa announced that he would bring eight members of the public along with him and pay for the entire journey.
He launched a documentary search for a new girlfriend to join him on the trip, before he canceled due to mixed feelings.
One of his posts became the most retweeted at the time after he promised financial incentives. Mr Maezawa said he will conduct another space event on his next trip.
The caption is media.
I bought a ticket to go to the moon.
Mr Maezawa is the first self-funded space tourist to travel to the International Space Station in more than a decade.
The only way to get to the International Space Station was on a Russian-made capsule called a Soyuz, but in the 2000s Russia started sending space tourists to the station, including the first non-astronaut to go into space.
The private space programme was suspended in 2010.
With the idea of space tourism gaining pace, it has started allowing paying customers like Mr Maezawa on its launches again.
In October, Russia sent a film director and an actor to the station to film scenes for a movie.
The likes of Richard Branson, Jeff Bezos and others are working on a separate sector of space exploration for rich clients.
Russia has not been able to separate the tourists from the professional cosmonauts. When a tourist goes into space on a Russian ticket, they have to be physically and technically prepared as well as professional astronauts, and go up with a team.
This requires a lot of time, money and other resources. The system is open to criticism that tourists are taking up valuable space which should be reserved for professionals only.
Russia is offering a much more realistic space experience to tourists.
The Karman line is the altitude at which the Earth atmosphere and outer space intersect. These trips allow the crew to experience zero gravity, enjoy impressive views, and return to Earth. A trip lasts 10 minutes and does not require training.
If a tourist goes into space in Russia, they have to stay for a minimum of one week. They must be in excellent health and they spend weeks preparing for the trip at the Baikonur launch pad.
They are billionaires.
Commercial space travel.
Asia.
Japan.
