To capture charged particles over an enlarged area to enhance sensitivity and resolution by placing a specifically shaped conical lens system that is in rotation symmetry with respect to the rotation axis of a toroidal spectrograph at the front stage of the spectroscope.

A conical electrostatic lens 6, which comprises four stages of lens systems in rotation symmetry with respect to the axis 1 of rotation, is placed at the front stage of a toroidal spectrograph. Electrons near an incidence slit for those coming from an electron generation source 9 are subjected to energy analysis by means of an electric field formed between outside and inside electrodes 2, 3, and are deflected by deflectors 10, 11 so that only those electrons within a specific electron energy range are detected and sorted out by an electron detector 12. The number and shape of the electrodes of the conical electrostatic lens 7 are selected appropriately to increase γ in the emission angle of the electrons, θ±γ, with respect to the rotation axis, to capture the electrons over a wide range and to enable the electrons to be focused near the incidence slit by the lens systems while being decelerated, resulting in enhanced sensitivity and resolution.