We have conducted a series of laboratory experiments to explore the bubble size distribution coupled with convection in a magma chamber. Boiling liquid heated from below exhibits two types of convection pattern depending on the viscosity. When the viscosity is sufficiently high, bubbles are distributed uniformly in the liquid, and the bubble size distribution becomes the power law type for large bubbles and exponential distribution for small bubbles. On the other hand, when the viscosity is low, bubbles are separated from the liquid layer, making a foam, and their size distribution is exponential for large bubbles and unimodal for small bubbles. These experimental results suggest that the bubble size distribution is determined whether the viscous drag of the magma is sufficiently high to trap bubbles.