Electrical discharge machine

An electrical discharge machine, comprising a critical angle detection device that detects a critical angle of a machining fluid in which a corrosion inhibitor is added, wherein

• the critical angle detection device comprises:
• a prism having an incident surface, a boundary surface, a reflection surface, and an emission surface;
• a light source that irradiates an incident light from the incident surface to the boundary surface which is a boundary between the prism and the machining fluid;
• an image sensor comprising a plurality of photodetectors that detect a reflection light reflected from the boundary surface and the reflection surface;
• an electrical circuit that calculates the critical angle by arithmetically processing output signals output from the plurality of photodetectors;
• and
• a slit that is arranged on an optical axis of the reflection light between the prism and the image sensor to block a scattered light.