A circuit board includes a substrate having an end surface, and a principal surface on which an electronic component is mounted, a first region, provided on the principal surface, and coated with a moistureproof agent, a second region, provided on the principal surface, and prohibited from being coated with the moistureproof agent, and a groove having two ends, formed in the principal surface, between the first region and the second region. The two ends of the groove reach the end surface of the substrate. The groove includes a guiding part, configured to guide the moistureproof agent overflowing from the first region into the groove, provided at a portion of the groove farthest away from the end surface.