In a server 10, a standard setting section 11 establishes a standard amount of deposits and a standard amount of money of a maintenance charge in advance; a first calculating section 12 calculates the maintenance charge by multiplying the standard amount of money by a ratio of a deposition level at returning to a standard deposition level; a memory section 13 stores in memory an engine operating time, the number of regeneration sequences performed, and a reduction coefficient table; a second calculating section 14 establishes a reduction coefficient based on the reduction coefficient table and reduces the maintenance charge after reduction by multiplying the maintenance charge calculated by the first calculating section 12 by the reduction coefficient; and a third calculating section 15 calculates an amount of refund by subtracting the value of the maintenance charge from the value of a maintenance guarantee.