A biological signal measuring system includes: first and second optical sensors attached to first and second body portions of a subject; a first average oxygen saturation acquiring section acquiring a first average value corresponding to an average value of the first oxygen saturation corresponding to an arterial oxygen saturation in the first body portion in a predetermined time period starting at a first time; a second average oxygen saturation acquiring section acquiring a second average value corresponding to an average value of the second oxygen saturation corresponding to an arterial oxygen saturation in the second body portion in the predetermined time period starting at a second time that is different from the first time; and a difference value acquiring section acquiring a difference value, to be displayed, between the first average value and the second average value.