Provided is a breast pump for the mother's milk, which makes it possible to obtain universal milk expression data without being affected by an individual difference, for example. The breast pump is composed of a collection container, a milk expressing member, and a negative pressure source for producing pulsated negative pressure. The milk expressing member is provided at an end with a teat cup and at an opposite end with a connector plug to the negative pressure source. The milk expressing member is divided by a partition plate. A flexible teat cup liner is replaceably fitted in the teat cup, and configured to be brought into contact with an areola and nipple of a breast.