In the type 0B string theory, we discuss the role of tachyon (T) and fixed scalars (ν,λ). The issue is to explain the difference between tachyon and fixed scalars in the D5_±-D1_± black hole background. For this purpose, we perform the semiclassical calculation. Here one finds a mixing between (ν,λ,T) and the other fields. Using the decoupling procedure, one finds the linearized equation for the tachyon. From the potential analysis, it turns out that ν plays a role of test field well, while the tachyon induces an instability of Minkowski space vacuum. But the roles of ν and T are the same in the near-horizon geometry. Finally we discuss the stability problem.