A gas collection device is installed in a toilet including a toilet seat and a toilet bowl. The gas collection device includes a flow path connected to a predetermined tank, and an introduction portion that introduces a sample gas into the flow path. The introduction portion is located between the toilet seat and the toilet bowl, or located inside or above the toilet seat. The introduction portion does not protrude toward an inside of the toilet bowl from a rim portion of the toilet bowl when located between the toilet seat and the rim portion. The introduction portion does not protrude toward the inside of the toilet bowl from the toilet seat when located inside or above the toilet seat.