To diagnose plasma in a plasma space, a plurality of floating probes are installed at a plurality of points, respectively, in a plasma space. An electron density ratio at each of the points is calculated by measuring a first probe current of each of the floating probes, the probe current including a DC component. A point ion density and a point electron temperature at each of the points are calculated by measuring a second probe current of each of the floating probes before the electron density ratio is calculated, the second probe current excluding the DC component.