Dynamic relaxation of Pd_42.5Ni_7.5Cu₃₀P₂₀ metallic glass (T_g～575 K) was measured in T = 153～563 K (27～98 % of T_g) with the angular frequency (ω) = 10⁻³～10² rad/s under the isothermal condition. By the time - temperature superimpose rule, master curves for storage (E'and loss (E'') moduli in the angular-frequency range with 16 orders of magnitude were successfully obtained. In addition to the α - relaxation and slow β - relaxation, which have been discusses in literatures, the present master curves exhibits another relaxation mode in the lower temperature region (< RT) with activation energy Q = 0.25 eV which is the fastest among three. These dynamic relaxations were characterized with the typical stretched exponent relaxation function, then, the processes for these relaxations were discussed thermodynamically from the activation energy and the attempt frequency.