We recently found a realistic compactification of the heterotic string theory which yields just the standard model gauge group with an extra U(1)B−L factor and no exotic matter. In my talk, I am going to explain our construction in detail. First we construct a torus fibered Calabi-Yau threefold with enough Wilson lines. Then we find a suitable E8 instanton to break the gauge group down to our extended standard model gauge group. Finally, we compute the low energy spectrum and find three generations without any exotic matter. Especially, doublets and triplets are split in the right way.