The present invention has an object to perform specimen charge measurement or focusing at a high speed and with high precision also for a specimen in which fixed charge and induced charge may be mixedly present.As one mode to achieve the object, there are proposed a specimen potential measuring method and a device to implement the method characterized in that when specimen potential information obtained by a first specimen potential measuring device disposed outside a specimen chamber or specimen potential information beforehand obtained is equal to or more than a predetermined threshold value or is more than the threshold value, measurement of specimen potential is selectively conducted by use of a second specimen potential measuring device in the specimen chamber.