A breast pump comprising: a variable pressure generating unit connected to an expressing unit for contact with a breast of a user, and alternately forming a negative pressure state below atmospheric pressure and a higher pressure than the pressure of the negative pressure state; and further comprising a control board configured to execute the overall operation of the breast pump; and a panel that has a display unit which displays required displays and an operating button by which the user can issue instructions; wherein the variable pressure generating unit includes: a power driving unit (40, 40A, 40B, 40C, 40D, 40E) including a stepping motor that is capable of rotating both normally and in reverse and a pair of linear moving bodies (44, 45) that are fixed to a surface of an endless belt at a predetermined distance from each other such that tip ends thereof face opposite directions; and a pair of negative pressure forming units (17, 17A, 17B, 18, 18A, 18B) each of which is provided with a respective linear moving body (44, 45) and is configured to create the negative pressure state by causing a respective elastic body (52, 52A, 52B) to deform in accordance with a cycle period formed by the power driving unit (40, 40A, 40B, 40C, 40D, 40E); wherein each negative pressure forming unit (17, 17A, 17B, 18, 18A, 18B) is coupled to the expressing unit; wherein the stepping motor is capable of modifying an amplitude of the cycle period and the cycle period during negative pressure formation as desired; wherein the linear moving bodies (44, 45) are configured to respectively reciprocate linearly in a rotation direction of a motor shaft of the stepping motor and create negative pressure by periodically increasing and reducing, in accordance with the cycle period, the size of a region (AS1, AS2) of the respective negative pressure forming unit (17, 17A, 18, 18A) in which the negative pressure forming unit (17, 17A, 18, 18A) generates negative pressure as the motor shaft repeatedly rotates normally and in reverse; and wherein the control board is capable of executing: an operating mode selected from a plurality of operating modes prepared in advance, an operating mode that performs an automatic operation in a single default operating pattern, and an operating mode in which the amplitude of the cycle period and the cycle period are selected and set individually as the operating button is operated in accordance with guidance on the display unit. a variable pressure generating unit connected to an expressing unit for contact with a breast of a user, and alternately forming a negative pressure state below atmospheric pressure and a higher pressure than the pressure of the negative pressure state; and further comprising a control board configured to execute the overall operation of the breast pump; and a panel that has a display unit which displays required displays and an operating button by which the user can issue instructions; a power driving unit (40, 40A, 40B, 40C, 40D, 40E) including a stepping motor that is capable of rotating both normally and in reverse and a pair of linear moving bodies (44, 45) that are fixed to a surface of an endless belt at a predetermined distance from each other such that tip ends thereof face opposite directions; and a pair of negative pressure forming units (17, 17A, 17B, 18, 18A, 18B) each of which is provided with a respective linear moving body (44, 45) and is configured to create the negative pressure state by causing a respective elastic body (52, 52A, 52B) to deform in accordance with a cycle period formed by the power driving unit (40, 40A, 40B, 40C, 40D, 40E); wherein each negative pressure forming unit (17, 17A, 17B, 18, 18A, 18B) is coupled to the expressing unit; wherein the stepping motor is capable of modifying an amplitude of the cycle period and the cycle period during negative pressure formation as desired; wherein the linear moving bodies (44, 45) are configured to respectively reciprocate linearly in a rotation direction of a motor shaft of the stepping motor and create negative pressure by periodically increasing and reducing, in accordance with the cycle period, the size of a region (AS1, AS2) of the respective negative pressure forming unit (17, 17A, 18, 18A) in which the negative pressure forming unit (17, 17A, 18, 18A) generates negative pressure as the motor shaft repeatedly rotates normally and in reverse; and wherein the control board is capable of executing: an operating mode selected from a plurality of operating modes prepared in advance, an operating mode that performs an automatic operation in a single default operating pattern, and an operating mode in which the amplitude of the cycle period and the cycle period are selected and set individually as the operating button is operated in accordance with guidance on the display unit.