A fluid flow detector includes a detector main body defining a fluid flow channel having a fluid flow detection channel positioned adjacent an upstream hole and a discharge channel positioned adjacent a downstream hole. A mobile body is positioned within the fluid flow channel and dimensioned to be movable along the fluid flow channel. The diameter of the fluid flow detection channel is smaller than the diameter of the discharge channel, and the maximum diameter of the outer peripheral edge part of the face of the fluid flow channel orthogonal to the direction of movement of the mobile body is slightly smaller than the diameter of the fluid flow detection channel. The upstream hole and the downstream hole are formed with a size and shape such that fluid can flow through the upstream and downstream holes without being obstructed by the mobile body.