The present invention pertains to a sensor for a diagnostic device using an optical fiber and a pH sensitive high molecular weight substance, which indicates the safety of foodstuffs or is used as a medical examination device. More particularly, the present invention relates to a sensor for a diagnostic device which detects a wavelength change of reflected light using an optical spectroscope to indicate a freshness of foodstuffs or an immune condition of a human body, including a light receiving optical fiber through which light is transferred from a light source to a pH sensitive high molecular weight substance. The sensor also includes a semi-permeable membrane film part which is filled with the pH sensitive high molecular weight substance so as to detect a pH change of a subject when the subject comes into contact with the pH sensitive high molecular weight substance, receives an output end of the light receiving optical fiber and an input end of an information transferring optical fiber, and includes a reflection member inserted therein so as to reflect the light passing through the light receiving optical fiber into the information transferring optical fiber. The information transferring optical fiber transfers data including the freshness of the foodstuffs or a health condition of the human body therethrough when the light subjected to a wavelength interference by the pH sensitive high molecular weight substance advances into the optical spectroscope. Alternatively, the sensor for the diagnostic device may includes a diagnostic kit insertion member instead of the semi-permeable membrane film part. The diagnostic kit insertion member is made of a material having excellent light transmittance, receives an output end of the light receiving optical fiber and an input end of the information transferring optical fiber, and has a diagnostic kit insertion groove for receiving a diagnostic kit. At this time, the diagnostic kit includes a semi-permeable membrane member and the pH sensitive high molecular weight substance. Accordingly, the sensor for the diagnostic device according to the present invention is advantageous in that it has a relatively short diagnostic time and excellent sensitivity and selectivity to a specific substance. Outer advantages are that the sensor may be repeatedly used many times and applied to various fields, and various diagnoses can conducted for a relatively short time through a simple operation in which various diagnostic kits are replaced with each other.