There are provided a drift calculation device capable of accurately calculating a drift by using a buffer of smaller capacity, and a light detection device provided with the same. Every time measurement intensity is input according to a predetermined cycle, data in a plurality of sum buffers 321 to 324 are updated based on at least one of the measurement intensity and the measurement time at that time. The sum buffers 321 to 324 are assigned respectively to a plurality of sum functions forming a coefficient included in a calculation formula for calculating a drift by using the least squares method. A drift is calculated by substituting the updated data in the plurality of sum buffers 321 to 324 in the calculation formula. Since it is not necessary to store all the measurement intensity input at the predetermined cycle, a drift is accurately calculated by a buffer of smaller capacity.