A differential pressure transmitting device detects, through two pressure guiding tubes, a differential pressure that is generated when that which is to be measured, such as a liquid, a slurry, or a gas, that has fluctuation in the pressure thereof, passes through a differential pressure generating mechanism. A pressure guiding tube blockage diagnosing device comprises: a rising/falling frequency detecting portion for segmenting the differential pressure values detected by the differential pressure transmitting device into a plurality of intervals, and for detecting, for each interval, the rising/falling frequency of the fluctuation of the differential pressure; and an evaluating portion for comparing the rising/falling frequency of the fluctuation to a specific threshold value to evaluate the state of blockage of the pressure guiding tubes.