  We introduce an etale fundamental group with modulus and construct a
reciprocity homomorphism from the Kato-Saito idele class group with modulus to
this fundamental group. This is the K-theoretic analogue of the reciprocity for
the cycle-theoretic idele class group with modulus due to Kerz-Saito, and plays
a central role in showing the isomorphism between the two idele class groups.
It also provides a new interpretation of the already known etale fundamental
group with modulus due to Deligne and Laumon.
