A method for forming an epitaxial wafer is provided as one enabling growth of a gallium nitride based semiconductor with good crystal quality on a gallium oxide region. In step S107, an AlN buffer layer 13 is grown. In step S108, at a time t5, a source gas G1 containing hydrogen, trimethylaluminum, and ammonia, in addition to nitrogen, is supplied into a growth reactor 10 to grow the AlN buffer layer 13 on a primary surface 11a. The AlN buffer layer 13 is so called a low-temperature buffer layer. After a start of film formation of the buffer layer 13, in step S109 supply of hydrogen (H2) is started at a time t6. At the time t6, H2, N2, TMA, and NH3 are supplied into the growth reactor 10. A supply amount of hydrogen is increased between times t6 and t7, and at the time t7 the increase of hydrogen is terminated to supply a constant amount of hydrogen. At the time t7, H2, TMA, and NH3 are supplied into the growth reactor 10.