  We report on the discovery of an Earth-sized transiting planet
($R_p=1.015\pm0.051\,R_\oplus$) in a $P=4.02$ day orbit around K2-415 (EPIC
211414619), an M5V star at 22 pc. The planet candidate was first identified by
analyzing the light curve data by the K2 mission, and is here shown to exist in
the most recent data from TESS. Combining the light curves with the data
secured by our follow-up observations including high-resolution imaging and
near infrared spectroscopy with IRD, we rule out false positive scenarios,
finding a low false positive probability of $2\times 10^{-4}$. Based on IRD's
radial velocities of K2-415, which were sparsely taken over three years, we
obtain the planet mass of $3.0\pm 2.7\,M_\oplus$ ($M_p<7.5\,M_\oplus$ at
$95\,\%$ confidence) for K2-415b. Being one of the lowest mass stars ($\approx
0.16\,M_\odot$) known to host an Earth-sized transiting planet, K2-415 will be
an interesting target for further follow-up observations, including additional
radial velocity monitoring and transit spectroscopy.
