A cover (43) made from a material such as a candy, once introduced into a subject, starts to melt in reaction with a substance in the subject. Soon the cover gets thin to expose an opening (41) to outside. The opening (41) is exposed only inside the subject. When a humidity sensor (42) and a body determining unit (32a) detect a predetermined level of humidity of saliva or the like, a driving controller (34) actuates an RF switch (32a) to turn an RF transmitter (23) into an operation mode. Thus, an image can be captured and transmitted at a position with the humidity detected by the humidity sensor (42).
