A sensor system and a sensor system using method improve operability of a placing operation of placing a sensor in a living body and improve user-friendliness of users. The sensor system has: a sensor member which has the sensor which detects analyte components of the living body and transmits a signal, and a retaining portion which retains the sensor; a needle assembly detachably attached to the sensor member and having a guide needle which guides insertion of the sensor to the living body; a signal processing circuit member detachably attached to the sensor member, and having an electronic circuit which processes the signal from the sensor; and a fixing member which fixes the sensor member to skin of the living body, and the sensor is placed in the living body in a state in which the signal processing circuit member is attached to the sensor member.