  We report the detection of the kinetic Sunyaev-Zel'dovich (kSZ) effect in
galaxy clusters with a 4.9 sigma significance using the latest 217 GHz Planck
map from data release 4. For the detection, we stacked the Planck map at the
positions of 30,431 galaxy clusters from the Wen-Han-Liu (WHL) catalog. To
align the sign of the kSZ signals, the line-of-sight velocities of galaxy
clusters were estimated with a machine-learning approach, in which the relation
between the galaxy distribution around a cluster and its line-of-sight velocity
was trained through a convolutional neural network. To train our network, we
used the simulated galaxies and galaxy clusters in the Magneticum cosmological
hydrodynamic simulations. The trained model was applied to the large-scale
distribution of the Sloan Digital Sky Survey galaxies to derive the
line-of-sight velocities of the WHL galaxy clusters. Assuming a standard
beta-model for the intracluster medium, we obtained the gas mass fraction in
R500 to be fgas,500 = 0.09 +- 0.02 within the galaxy clusters with the average
mass of M500 ~ 1.0 x 10^14 Msun/h.
