A method comprising providing a reflective region at a far side of tissue of a subject is provided. The polymer electrolyte is produced by mixing proton-conducting sulfonated polyethersulfone C1, sulfonated polyphenylene sulfide C2 or sulfonated poly C3 having a sulfonic acid group A as a protic acid group with 1,4-benzenedimethanol B as a crosslinking agent having a methylol group and heat-treating the mixture so that a reaction can be carried out. The weight portion is formed so that the proportion of the length of the weight portion to the length from the root to the tip end side in a longitudinal direction of the resonating arm is within a range of 35% to 41%.