Abstract: The invention relates to a manually operated pump able to dispense liquid or creamy substances at a predetermined constant pressure. The pump comprises a hollow main body carrying a dip tube and fixable to the mouth of a container containing the substance to be dispensed. The cavity of the main body houses a movable piston and a stem which are sealedly connected together. The piston is sealedly slidable along a cylindrical surface of the main body and is urged by a spring which maintains the stem thrust against a stop on the main body, with a stem appendix projecting to the outside. Between the stem and the piston a chamber is defined which houses in a sealedly mobile manner a valving member which is urged by a spring against an annular ridge projecting from the opposing surface of the piston. The two springs are isolated from the path taken by the substance to be dispensed, and the delivery pressure of the substance can be easily predetermined.