Hiroshi first started off-road racing in 1979 and accumulated impressive results in domestic Japanese events before debuting in international events. He attempted his first Paris-Dakar in 1987.
In 1990 he took an impressive T2 category win (10th overall). His list of impressive finishes in the Dakar continued : he finished 4th overall in 1994 and in 1997/1998 he took two consecutive 4th outright places. He took 6th overall in 1999 and 2000, steadily building an impressive record in this classic event.
In November 2000 Masuoka went to the Baja 2000, the most famous and toughest off-road event in America. The off-road event held in Mexico is very different from European rallies. Indeed, the Baja 2000 requires that competitors drive their machines over 2,500km non-stop! Masuoka teamed up with 3 American drivers to cross the Mexican Baja California peninsula in a production-based Mitsubishi Montero Sport/Challenger.
Masuoka and his team-mates succeeded in taking the Montero Sport/Challenger to victory in class 3I, completing the race in a little more than 55 hours, an incredible achievement considering that many doubted they would even finish on their first attempt at the Baja. The American challenge provided Masuoka with valuable experience for his attempt to conquer the 2001 Dakar, a feat that he came within two and a half hours of pulling off after 21 days of competition, unfortunately finishing second overall.