To provide a connection structure of a circuit board capable of obtaining stable ground connection.

A first circuit board 20 or 60 has a first ground pattern 23 or 63. A second circuit board 40 or 80 has a second ground pattern 43 or 83. The first circuit board 20 has a concave-convex portion 25 provided on a side end surface. The second circuit board 40 has a second concave-convex portion 45 fitting to the first concave-convex portion 25. The first ground pattern 23 or 63 or the second ground pattern 43 or 83 formed on at least one side end surface of the first circuit board 20 or 60 or the second circuit board 40 or 80 is connected to the second ground pattern 43 or 83 of the second circuit board 40 or 80 or the first ground pattern 23 or 63 of the first circuit board 20 or 60.