PROBLEM TO BE SOLVED: To provide an endoscope system in which image signals of a plurality of colors can be aligned highly correctly even when an oxygen saturation level is calculated on the basis of the image signals of the colors acquired by a sequential method.SOLUTION: A subject is sequentially irradiated with a first white light generated by excitation of a phosphor with a first laser beam having a center wavelength of 473 nm and a second white light generated by excitation of the phosphors with a second laser beam having a center wavelength of 445 nm in separated two frames. Reflection images of the subject are sequentially picked up for each frame by a color image pickup device. Positional shift amount (shift amount) between the frames is obtained from a green signal G1 of a first frame and a green signal G2 of a second frame. Based on the shift amount, a blue signal B1 of the first frame is moved to be aligned with an image of the green signal G2 and a red signal R2 of the second frame. An oxygen saturation image representing an oxygen saturation level of hemoglobin in blood is produced from the aligned signals B1, G2 and R2. The oxygen saturation image which has been produced is displayed on a display unit.【課題】順次方式で得られた複数の色の画像信号に基づいて酸素飽和度を算出する場合であっても、各色の画像信号間の位置合わせを精度良く行う。【解決手段】中心波長４７３ｎｍの第１レーザ光により蛍光体で励起発光する第１白色光と、中心波長４４５ｎｍの第２レーザ光により蛍光体で励起発光する第２白色光とを、２フレームに分けて、被検体に順次照射する。被検体の反射像は、カラーの撮像素子でフレーム毎に順次撮像される。１フレーム目の緑色信号Ｇ１と２フレーム目の緑色信号Ｇ２から、フレーム間の位置ずれ量（ずれ量）を求める。この位置ずれ量に基づいて、１フレーム目の青色信号Ｂ１を２フレーム目の緑色信号Ｇ２及び赤色信号Ｒ２に位置合わせする。位置合わせがされた信号Ｂ１，Ｇ２，Ｒ２に基づいて、血中ヘモグロビンの酸素飽和度を画像化した酸素飽和度画像を生成する。生成した酸素飽和度画像は、表示装置に表示される。【選択図】図６
