EXHIBIT 32.2

                   CERTIFICATION OF PERIODIC FINANCIAL REPORTS

I, Robert J. Messey, Executive Vice President and Chief Financial Officer of
Arch Coal, Inc., certify, pursuant to 18 U.S.C. Section 1350, as adopted
pursuant to Section 906 of the Sarbanes-Oxley Act of 2002, that:

    (1) the Annual Report on Form 10-K for the year ended December 31, 2004 (the
"Periodic Report") which this statement accompanies fully complies with the
requirements of Section 13(a) or 15(d) of the Securities Exchange Act of 1934
(15 U.S.C. 78m or 78o(d)); and

    (2) information contained in the Periodic Report fairly presents, in all
material respects, the financial condition and results of operations of Arch
Coal, Inc.

                                           /s/ ROBERT J. MESSEY
                              --------------------------------------------------
                                             Robert J. Messey
                             Senior Vice President and Chief Financial Officer

Dated March 10, 2005