A residual charge amount estimation method including the steps of: providing a radiation image detector including pixels, scanning lines, and data lines, each pixel having a collection electrode, a capacitor, and a TFT switch; detecting a leak current flowing out through each data line with the TFT switches being switched OFF; reading out an image signal flowing out through each data line by sequentially switching ON the TFT switches connected to each scanning line on a scanning line-by-scanning line basis; calculating an average leak current per unit saturated pixel from the leak current detected from at least one of the data lines on which a saturated pixel having a saturated value of the image signal is found and the number of saturated pixels; and estimating the amount of residual charges remaining in the unit saturated pixel based on the average leak current.