PURPOSE: To purely measure only a component by an electrophoresis, by a method wherein, using a projection optical system having an optical axis in parallel to a stationary surface formed in an electrophoresis pipe, an image in a stationary surface is formed at the center of a mask of a detecting means for suspending particle image.

CONSTITUTION: An electrophoresis pipe 1 is filled with solution, the inner surface of the pipe 1 is charged into a negative, and positive ion gathers at a surface which makes contact with the inner wall of the pipe 1 filled with solution. If an electric field is caused to act in the axial direction of the pipe 1, solution at a center flows to form a zero layer in a cylinder as shown in a chain line through the movement of liquid. If the pipe 1 is illuminated by a light source 3 from the side, an image of a stationary surface by an objective 5 is reflected (6) and is formed on a grating 7. A part except the image on the stationary surface is cut by a mask 8 at the front of the grating 7, a transmission light through the grating 7 is detected by a photo detector 9, and an electrophoresis speed of a cell can be detected from a frequency of an alternating current component of a detecting output. Through looking through an eyepiece 11, an inclination of a parallel plane plate 12 is adjusted to cause the image on the pipe wall to coincide with an index 14, and this forms the image on the stationary surface on the center of the mask 8.