A pilot type solenoid valve including: a pilot valve unit having a fixed core and a movable core; and a passage block unit formed with a valve seat with which a valve element will come into or out of contact; wherein the passage block unit has a rectangular parallelepiped shape including a pair of wide opposite surfaces (3a, 3b) and four side surfaces surrounding the wide opposite surfaces, the four side surfaces include a first surface on which the pilot valve unit is attached, and a second surface formed with at least either an inlet port or an outlet port, the valve element is a diaphragm valve and arranged in parallel to the pair of wide opposite surfaces (3a, 3b), the passage block unit includes a block body forming a valve chamber around the valve seat, and a flat cover of a flat plate shape, the flat cover includes a recess forming a back chamber of the diaphragm valve, the flat cover has a flat cover communication path formed to provide communication between the recess and a common port of the pilot valve unit, wherein the diaphragm valve divides the valve chamber from the back chamber, and the block body is formed with an inlet passage allowing communication between the valve chamber and the inlet port, and an outlet passage allowing communication between a valve hole defined by an opening of the valve seat and the outlet port, characterized in that the valve hole forms a valve hole chamber having a bottom communicated with the outlet passage formed in a slit shape, the valve chamber is formed as a recess having a nearly cylindrical shape, wherein the valve chamber and the valve hole chamber are concentric to each other, and a left end of a bottom of the valve hole chamber is communicated with the outlet passage, and a right end of the valve chamber is communicated with the inlet passage, the inlet passage having a square cross-sectional shape. a pilot valve unit having a fixed core and a movable core; and a passage block unit formed with a valve seat with which a valve element will come into or out of contact; wherein the passage block unit has a rectangular parallelepiped shape including a pair of wide opposite surfaces (3a, 3b) and four side surfaces surrounding the wide opposite surfaces, the four side surfaces include a first surface on which the pilot valve unit is attached, and a second surface formed with at least either an inlet port or an outlet port, the valve element is a diaphragm valve and arranged in parallel to the pair of wide opposite surfaces (3a, 3b), the passage block unit includes a block body forming a valve chamber around the valve seat, and a flat cover of a flat plate shape, the flat cover includes a recess forming a back chamber of the diaphragm valve, the flat cover has a flat cover communication path formed to provide communication between the recess and a common port of the pilot valve unit, wherein the diaphragm valve divides the valve chamber from the back chamber, and the block body is formed with an inlet passage allowing communication between the valve chamber and the inlet port, and an outlet passage allowing communication between a valve hole defined by an opening of the valve seat and the outlet port, characterized in that the valve hole forms a valve hole chamber having a bottom communicated with the outlet passage formed in a slit shape, the valve chamber is formed as a recess having a nearly cylindrical shape, wherein the valve chamber and the valve hole chamber are concentric to each other, and a left end of a bottom of the valve hole chamber is communicated with the outlet passage, and a right end of the valve chamber is communicated with the inlet passage, the inlet passage having a square cross-sectional shape.