A highly reliable light-emitting element that shows a novel degradation curve when driven is provided. The degradation curve of a light-emitting element with a long lifetime is represented by a function composed of two exponential functions, which is a linear combination of an exponential function expressing an initial degradation component and an exponential function expressing a long-term degradation component. The initial degradation component is represented by a stretched exponential function, and the long-term degradation component is represented by a single exponential function. The light-emitting element is highly reliable when formed to show a degradation curve represented by the function composed of two exponential functions. In other words, the function composed of two exponential functions in one embodiment of the present invention is capable of accurately fitting the degradation curve of the highly reliable light-emitting element.