An electrochemical sensor includes a base plate including one end portion and another end portion, an electrode portion formed on the one end portion of the base plate, a connecting portion, formed on the another end portion of the base plate, for electrically connecting the electrode portion to a monitoring instrument, and an attaching portion formed on the another end portion, the attached portion being employed for attaching the another end portion to the monitoring instrument in a state where the one end portion is enabled to swing relatively to the monitoring instrument.