The detailed nature of Type Ia supernova progenitors has a profound impact on the thermonuclear supernova explosions and their utility in cosmology. Metallicity of the Type Ia progenitor is of particular interest because it is a key parameter to reveal the progenitor's age that may discriminate two populations of Type Ia supernovae (prompt vs delayed). Atomic X-ray line emission from trace elements Mn and Cr in young Type Ia supernova remnants provides an excellent opportunity to estimate the metallicity of the progenitor star. We have performed a deep Suzaku observation of Kepler's supernova remnant to reveal its progenitor's metallicity. Our preliminary results from this Suzaku Key Project indicate that Kepler's supernova was an explosion of a white dwarf with high metallicity (i.e., 4 times above the Solar), thus likely a young, prompt population of Type Ia supernova.