To automatically and efficiently process prepayment while surely dealing with consumption fund and automatic withdrawals, thereby reducing a payback period or total interest costs.

For an account 1 related to an overdraft-based mortgage, this loan account processing method includes a step in which a computer having a storage device 54 for storing various information and either a communication control device 56 or a control device 58 for accepting money received and money paid in relation to overdrafts within an overdraft limit 20 deducts the money received from the principal 8 according to a date when the money received is accepted; a step in which the computer adds the money paid to the principal 8 according to a date when the money paid is received; a step in which on each date the computer adds an original principal on the date by the integrated number; a step in which the computer deducts a minimum monthly payment from the overdraft limit 20 on the date of payment; and a step in which the computer calculates an interest 38 on the basis of the integrated number and an agreed interest and adds the interest 38 to the principal 8 and the interest 38 to the overdraft limit 20.

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