To provide a method of observing a living unit under an electron microscope which can perform observation of the living unit in a biological environment with the electron microscope.

A biological environment 11 is provided inside a sample chamber 91 in the electron microscope 90 and a living unit 18 is prepared in the environment 11 with a predetermined environmental condition which is adjusted so that the unit can retain its basic physiology. Observation windows 12 facing each other are formed at upper and lower sides, respectively, of the biological environment 11. The method include a step of adjusting the living unit 18 to have two or more objects which can bear different critical charge densities, respectively, and a step of irradiating a predetermined area of the living unit 18 with a particle beam of a predetermined intensity for a predetermined period through the observation windows 12 to focus an image on an imaging device of the electron microscope 90. The predetermined charge densities are obtained as products of the predetermined intensity of the particle beam and the predetermined period and are smaller than or same as the critical charge density of the object in the irradiated area of the living unit 18.