We used red clump stars to measure the surface mass density of the Galactic disk in the solar neighbourhood. High resolution spectra of red clump stars towards the NGP have been obtained with the ELODIE spectrograph at OHP for Tycho-2 selected stars, and nearby Hipparcos counterparts were also observed. We determined their distances, velocities, and metallicities to measure the gravitational force law perpendicular to the Galactic plane. As in most previous studies, we applied one-parameter models of the vertical gravitational potential. We obtained a disk surface mass density within 1.1 kpc of the Galactic plane, Sigma_1.1 kpc = 64±5 {M}&odot; pc-2, with an excellent formal accuracy, however we found that such one-parameter models can underestimate the real uncertainties. Applying two-parameter models, we derived more realistic estimates of the total surface mass density within 800 pc from the Galactic plane, Sigma0.8 kpc = 57-66 {M}&odot; pc-2, and within 1.1 kpc, Sigma_1.1 kpc = 57-79 {M}&odot; pc-2. This can be compared to literature estimates of 40 {M}&odot; pc-2 in stars and to 13 {M}&odot; pc-2 in the less accurately measured ISM contribution. We conclude that there is no evidence of large amounts of dark matter in the disk and, furthermore, that the dark matter halo is round or not vey much flattened. A by-product of this study is the determination of the half period of oscillation by the Sun through the Galactic plane, 42±2 Myr, which cannot be related to the possible period of large terrestrial impact craters 33-37 Myr.< Réduire