This paper is concerned with the development of an active palpation sensor for detecting prostatic cancer and hypertrophy. The receptor of the sensor is a polyvinylidene fluoride (PVDF) film placed on the surface of a sponge rubber layer. It is mounted on a linear z-translation bar and inserted into the examinee's rectum whilst being protected by a medical rubber glove. After being positioned to face the prostate gland, the sensor probe is driven sinusoidally at about 50 Hz, with a peak-to-peak amplitude of 2 mm. The voltage signal from the PVDF film is integrated over the sampling period and is used as the output of the sensor for extracting the features of the collected data. The evaluation of the stiffness by the sensor on 27 normal and unhealthy prostate glands are compared with the results of diagnosis by the doctor's palpation. It is shown that the output of the sensor becomes greater with an increase of the stiffness of the prostate gland, which has a good correlation with the doctor's evaluation on the stiffness. Further results on the laboratory test reconfirm that the present sensor well discriminates the stiffness of the prostate glands in vivo and non-invasively.