Dielectric behaviors of a binary mixture composed of TX100 (a nonionic surfactant) and formamide (FA) at different surfactant concentrations and varying temperature were investigated over a frequency range from 40 Hz to 110 MHz. One relaxation appeared around gigahertz is considered to be from the contribution of two types of FA; one is "free FA", which has no interaction with surfactant, and the other is "associated FA", which can interact with surfactant. Conductivity was used to determine the number of associated FAs, and the result indicates that each ethylene oxide (EO) segment binds to one FA molecule. The dipole moment of the associated FA was calculated by using Cavell equation, and it is smaller than those of bulk FA, while the dipole rotation time of associated FA is higher than that of bulk FA. This suggests that the dynamics of associated FA is restricted by the hydrophilic chain of surfactant. The thermodynamic parameters, obtained from the temperature depedences of the relaxation times, revealed that in dilute TX100-FA solution the interaction FA with EO segment of surfactant is weaker compared with the FA-FA hydrogen bond. This work also demonstrated that the dynamics of associated FA is quite similar to that of hydration water.