PURPOSE: To increase enlarging ratio, by making tension act on both ends of an expanding and contracting body through a body, which is not expanded or contracted, fixing both ends to the connecting bodies under the state a string shaped body whose length is unchanged is slackened, and applying an external force to the central point in the lateral direction.

CONSTITUTION: A fixed body 2 is linked to a print B of an expanding and contracting body (hair) 3, which is displaced by humidity. A movable body 4, a spring 5 and a fixed body 1 are linked to a point A. A string shaped or ribbon shaped body 6, whose length is not changed by humidity, is attached to a part between a point A' of a body 4 and a point B' of a body 2. A central point C is pulled in the direction CE. In this mechanism, an expression shown in the Figure is realized. Change dL' of the expanding and contracting body 3 is enlarged by 1/tan θ. The value is detected as a change dλ in distance λ between D and C.