The magnetic-field dependence of optical reflectivity [$R(omega)$] and optical conductivity [$sigma(omega)$] spectra of the ideal type-I Weyl semimetal TaAs has been investigated at the temperature of 10 K in the terahertz (THz) and infrared (IR) regions. The obtained $sigma(omega)$ spectrum in the THz region of $hbaromegaleq15$ meV is strongly affected by the applied magnetic field ($B$): The Drude spectral weight is rapidly suppressed and an energy gap originating from the optical transition in the lowest Landau levels appears with a gap size that increases in proportion to $sqrt{B}$, which suggests linear band dispersions. The obtained THz $sigma(omega)$ spectra could be scaled not only in the energy scale by $sqrt{B}$ but also in the intensity by $1/sqrt{B}$ as predicted theoretically. In the IR region for $hbaromegageq17$ meV, on the other hand, the observed $R(omega)$ peaks originating from the optical transitions in higher Landau levels are proportional to linear-$B$ suggesting parabolic bands. The different band dispersions originate from the crossover from the Dirac to the free-electron bands.
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