The objective of the present invention is to embody a convenient and reliable image sensor of an auto trigger scheme. The present invention provides a driving method for an image sensor in which pixels are arranged in a matrix form along row lines and column lines and which has a waiting mode and an image obtaining mode, the driving method for an image sensor comprising the steps of: when sequentially selecting a row line, applying a gate signal, and reading out a data signal for each frame in the waiting mode, calculating a variation between the data signal of the currently selected row line and the data signal of the same row line in the previous frame; and if the variation is greater than or equal to a threshold value, determining that light has been radiated.