To provide a high speed rotation restraining mechanism of a windmill for power generation, capable of automatically performing operation and a release of a brake, and capable of realizing this operation with a simple structure without using electricity.

A brake means 7 has bimetal 9 as an operation element operating by reacting to heat. The bimetal 9 is equipped on the rotary side 2 of the windmill 1 so as to transmit heat generated by a power generation part 6 of the windmill 1; performs deforming operation by the heat generated from the power generation part 6 in high speed rotation of the rotary side 2; brakes the rotary side 2 by generating friction by contact between the rotary side 2 and the fixed side 8 of the windmill 1; and releases a brake by cutting off the contact by returning by a temperature drop in the power generation part 6.

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