A calculation apparatus in which a controlling unit performs a first storing process for storing a first value in a storing unit; a first displaying process for executing operation on a second value, using the first value to obtain and display a first and second output value on a displaying unit; a second displaying process for executing operation on a third value, using the first value to obtain and display a third and fourth output value on the displaying unit; a second storing process for adding the third output value to the first output value and the fourth output value to the second output value to obtain and store a first sum and a second sum in the storing unit; and a sum displaying process for displaying the first and second sum stored in the storing unit on the displaying unit in response to operation of the key unit.