A recess (5a) in the corner direction and recesses (5b) in side directions are formed in each connecting pad (5A) located at a corner of a lower surface-side of an insulating base 2 having groove-shaped recesses (6) in the periphery, and groove-shaped recesses (6a and 6b) in the corner and side directions are formed in each corner portion (2A) of the insulating base 2 corresponding to the connecting pad (5A). Connecting pads (5) of an electronic apparatus in which an electronic component is mounted on the insulating base 2 are mounted on an external electrical circuit board by using a solder. A solder (31) melted during the solder-mounting adheres onto the groove-shaped recesses (6a and 6b) in the corner and side directions of the corner portion (2A) of the insulating base 2 and thus solder fillets are formed in the groove-shaped recesses (6a and 6b). Thus, solder bonding strength in the corners where external force is likely to work can be increased and the connecting pads of the wiring board can be firmly bonded to the lead conductors of the external electrical circuit board by using a hard and brittle lead-free solder.