PURPOSE:To measure not only gaseous pressure but also liquid pressure and to measure the same highly accurately with a simple constitution, by introducing the fluid to be measured into the inner side of a power detector and varying the resonance frequency of a oscillator by varying the torsion oscillation of the annular vibrator. CONSTITUTION:A power detector 1 is constituted by a main body 11 consisting of a thick-wall cylindrical body 12, a thin-gauge disk 13 closed at its one end and an annular vibrator 14 attached to the outside of the disk 13 and a driving means 15 such as piezo-electric element etc. attached to the inside of a base part of the vibrator 14. The vibrator 14 is formed in funnel shape expanding at the upper part and it is made of a magnetic material and it is twisted previously by an angle phi. When an output signal of an oscillator 2 is inputted into the means 15, the vibrator 14 twisted previously by an angle phi to the outward generates a twist oscillation by an oscillation mode as shown by a brocken line. Resonance frequency of the vibrator 14 is varied by varying said twist oscillation and the pressure of a fluid to be measure is measured by this variation.