A laser device comprising: a first laser medium and a second laser medium each have a first surface (12a, 22a) and a second surface (12b, 22b) opposite to the first surface (12a, 22a), and each receive input of excitation light and seed light from the first surface side to amplify the seed light; a holder that holds the first laser medium and the second laser medium; and wherein a first mirror (M1, M2) that reflects at least the seed light is provided on the second surface (12b, 22b) of each of the first laser medium and the second laser medium, and the first laser medium and the second laser medium are disposed at relative positions in which the seed light reflected by the first mirror of the first laser medium is further reflected by the first mirror of the second laser medium, characterized in that the laser device further comprises a pair of cooling units configured to respectively cool the first laser medium and the second laser medium according to change in volume of a refrigerant, and configured to cause the first laser medium and the second laser medium to vibrate due to the change in volume of the refrigerant, wherein the holder integrally holds the first laser medium and the second laser medium so that the relative position is maintained and the vibration of the first laser medium and the second laser medium is synchronized, wherein a heat sink (14, 24) is provided on the second surface (12b, 22b) of each of the first laser medium and the second laser medium, and thermally connected through the respective first mirror (M1,M2) to the respective first laser medium and the second laser medium, wherein the holder holds the heat sinks (14, 24), thereby holding the first laser medium and the second laser medium, wherein the first laser medium and the second laser medium are each thermally connected to the cooling unit via a flexible heat conductive member, wherein the flexible heat conductive member has one end attached to the cooling unit and the other end attached to the heat sink (14,24), and wherein the cooling unit is a refrigerator that is configured to cool the first laser medium and the second laser medium according to periodic compression and expansion of the refrigerant. a first laser medium and a second laser medium each have a first surface (12a, 22a) and a second surface (12b, 22b) opposite to the first surface (12a, 22a), and each receive input of excitation light and seed light from the first surface side to amplify the seed light; a holder that holds the first laser medium and the second laser medium; and wherein a first mirror (M1, M2) that reflects at least the seed light is provided on the second surface (12b, 22b) of each of the first laser medium and the second laser medium, and the first laser medium and the second laser medium are disposed at relative positions in which the seed light reflected by the first mirror of the first laser medium is further reflected by the first mirror of the second laser medium, characterized in that the laser device further comprises a pair of cooling units configured to respectively cool the first laser medium and the second laser medium according to change in volume of a refrigerant, and configured to cause the first laser medium and the second laser medium to vibrate due to the change in volume of the refrigerant, wherein the holder integrally holds the first laser medium and the second laser medium so that the relative position is maintained and the vibration of the first laser medium and the second laser medium is synchronized, wherein a heat sink (14, 24) is provided on the second surface (12b, 22b) of each of the first laser medium and the second laser medium, and thermally connected through the respective first mirror (M1,M2) to the respective first laser medium and the second laser medium, wherein the holder holds the heat sinks (14, 24), thereby holding the first laser medium and the second laser medium, wherein the first laser medium and the second laser medium are each thermally connected to the cooling unit via a flexible heat conductive member, wherein the flexible heat conductive member has one end attached to the cooling unit and the other end attached to the heat sink (14,24), and wherein the cooling unit is a refrigerator that is configured to cool the first laser medium and the second laser medium according to periodic compression and expansion of the refrigerant.