Provided are an MEMS device, a head, and a liquid jet device in which substrates are inhibited from warping, so that a primary electrode and a secondary electrode can be reliably connected to each other. Included are a primary substrate 30 provided with a bump 32 including a primary electrode 34, and a secondary substrate 10 provided with a secondary electrode 91 on a bottom surface of a recessed portion 36 formed by an adhesive layer 35. The primary substrate 10 and the secondary substrate 30 are joined together with the adhesive layer 35, the primary electrode 34 is electrically connected to the secondary electrode 91 with the bump 32 inserted into the recessed portion 36, and part of the bump 32 and the adhesive layer 35 forming the recessed portion 36 overlap each other in a direction in which the bump 32 is inserted into the recessed portion 36.