We describe Fourier domain optical coherence tomography equipped with a novel linear-in-wavenumber spectrometer. The presented device linearizes the spectral dispersion of the spectrometer in wavenumber using a specifically designed prism. The spectral linearity in wavenumber makes numerical interpolation into wavenumber unnecessary, reduces computing time, and furthermore results in improvement of the falloff of signal with image range inherent to frequency-domain optical coherence tomography imaging. Experiments demonstrate the improvement of the falloff and agree with the expected results from simulation.