An electrochemical detector is an electrochemical detector for detecting a substance in a liquid by generating a redox cycle, the electrochemical detector comprising: a first working electrode having a first electrode surface, a second working electrode having a second electrode surface, and a plurality of insulating spacer particles, wherein the first and second electrode surfaces are placed so as to face each other so that an electric field is formed between the first and second electrode surfaces, and the plurality of spacer particles are placed along the first and second electrode surfaces so as to separate the first and second electrode surfaces from each other.